Introduction

There are currently some 14 genera recognised in the orthurethran land snail family Cerastidae. Their greatest diversity is found in the Afro-tropical zone, but the family extends eastwards into the Indian subcontinent and beyond, and one genus, Amimopina, occurs in Australia and New Guinea (Solem 1964), Thailand (Sutcharit et al. 2010), and probably also Cambodia (Mordan 1992). A few species have become widely dispersed in many of the islands of the Indo-Pacific by human agency.

The family Cerastidae Wenz, 1923 was previously included in the Enidae sensu lato (Enoidea sensu Bouchet and Rocroi 2005), and first recognised as a distinct group of orthurethran land snails by Watson (1920: 22), who stated that there can be little doubt that Pachnodus and its allies should be placed in a separate subfamily from the Palaearctic forms, or perhaps even in a distinct family. The phylogenetic relationship of several cerastid genera was subjected to a morphology-based cladistic analysis by Mordan (1992), and the monophyletic nature of the family has recently been supported by molecular evidence (Wade et al. 2006).

The principal distinguishing anatomical feature of cerastids is the excretory system which, whilst generally being of the normal orthurethran form comprising an elongate kidney with a straight primary ureter running directly towards the pneumostome, has developed what Solem (1964: 115) has termed a ‘pseudo-sigmurethrous’ secondary ureter. This takes the form of a fold or tube running from the renal orifice back along the kidney to the top of the lung, and usually a partial or complete fold running back along the rectum, towards the pneumostome, the whole ureteric anatomy mirroring that of true sigmurethrous Stylommatophora. Partial or complete closure of the renal fold to form a tube is known in Edouardia, Amimopina, Rachistia and Limicena (Solem 1964, Mordan 1992, 1998), but none of these has a partly, let alone completely closed rectal fold. Presumably these anatomical structures have the effect of directing the excretory products of the kidney along a pathway in which there can be some water resorbtion, and are consistent with the group being broadly xerophilic in its habits.

Here we present an account of a new genus and species of cerastid from the Lesser Himalaya of Central Nepal. Along its E-W axis, Nepal is divided into five east-west parallel physiographic elevational regions i.e. from south (lower) to north (higher): 1) Tarai (southern flat belt from 67 m – 300 m), 2) Shiwalik (from 300 m – 1000 m), 3) Mid Hill (1000 m – 3000 m), 4) Mid Mountain (3000 m – 5000 m) and 5) High Himalaya (> 5000 m). Our surveys were conducted in these five zones in Central Nepal but the new genus was reported only from the Shiwalik range (Figure 1). The type material has been deposited at the museum of the Central Department of Zoology, Tribhuvan University (CDZTU), Kathmandu, Nepal.

Shell measurements are expressed in mm as follows: shell height × max. shell width × min. shell width. Abbreviation for internal parts used are; AG- albumen gland, ATR- atrium, AU- auricle, E- epiphallus, EC- epiphallar caecum, GS- gametolytic sac, HD- hermaphrodite duct, KI- kidney, P- penis, PI- pilaster, PNE- pnerumostome, PR- penis retractor, REU- renal branch of secondary ureter, RT- rectum, RTU- rectal branch of secondary ureter, SO- spermoviduct, UO- ureteric orifice, VD- vas deferens and VE- ventricle.

Taxonomic treatment Discussion

Darwininitium is of particular interest on two counts: firstly its distribution represents a considerable extension of the known range of the Cerastidae, and secondly it exhibits the most advanced development of the pseudosigmurethrous condition yet found in the family, or indeed in any orthurethran species.

A very short partial renal fold is found in some genera in a few families of Orthurethra, for example Acanthinula (family Acanthinulidae), but a secondary ureteric structure is by far the most clearly and widely developed in the Cerastidae. Pseudosigmurethry was first described in the Australian cerastid Amimopina macleayi (Brazier) by Solem (1964). This monotypic genus has a fully closed renal tube running along the side of the kidney, from the renal pore to almost the top of the lung. It terminates as a short, so-called ‘renal ridge’. There is a short open gap across the top of the lung cavity before a rectal fold, open along its entire length, runs almost the full length of the rectum as far as the pneumostomal complex. In Darwininitium the rectal fold is fused to the rectum along its entire length, meaning that there is a continuous tube running from the tip of the kidney to its opening adjacent to the anus and pneumostome, mirroring more closely than previously known the situation in the true Sigmurethra.

The position of Darwininitium within the Cerastidae is confirmed by the well-developed pseudosigmurethry, as well as by the highly characteristic darkly pigmented spongy tissue lining the atrium, not known in any other orthurethran family, and the prominent pallial venation (Mordan 1992). It differs from the Enidae, in which the family was earlier included, in lacking hermaphrodite duct diverticulae and a mantle gland.

In addition to its unique pallial anatomy, Darwininitium is immediately separable from other cerastid genera for which there is anatomical information, by the absence of a penial appendix: an elongate, tubular structure, highly differentiated into several distinct regions along its length, inserting well below the point of insertion of the vas deferens, and with its own retractor muscle system. The appendix is found in all other cerastid genera, and occurs commonly throughout the Orthurethra. Indeed, it is difficult to associate Darwininitium phylogenetically with any of the existing cerastid genera. Mordan (1992, 1998) has analysed the anatomy of the reproductive, alimentary and pallial systems of 11 genera of cerastid and produced a morphological phylogeny based largely on these characters. One clade, which includes Rhachistia, Edouardia, Amimopina and Limicena, has a renal fold which is either partly or wholly fused along its length to form a tube, and therefore most closely approaches the condition found in Darwininitium, but none of these shows any fusion of the rectal fold, let alone complete fusion in the form of a tube along its entire length. However, this clade is further characterised by an extremely short gametolytic duct, and a relatively long and very narrow flagellum which inserts on the caecum at the head of the penis, level with or above the point of insertion of the vas deferens, characters not found in Darwininitium which has a rather long gametolytic duct and no flagellum. The shell of Darwininitium is closest in shape to certain carinated species of Edouardia, but the pigmentation of white flecks on a dark-brown background is unlike any other cerastid.

Biogeographically this new record provides a significant range extension to the family Cerastidae, Nepal lying significantly further north of the previous known distribution. Mordan (1992; Fig. 1) summarised the distribution of the Cerastidae, which is centred on the Afro-tropical region with by far the greatest diversity being found in southern Africa and Arabia. There are outliers in the Indian subcontinent, Cambodia, Thailand, Australia, South-Eastern New Guinea, and now Nepal. Additionally a few species such as Rhachistia histrio (Pfeiffer) have spread into many other islands of the Indo-Pacific, and were considered by Solem (1959) at least in some cases to have been dispersed by human agency. In the Indian subcontinent cerastids are restricted to a band running down the west coast and including Sri Lanka, and an area around the Ganges delta.

The detailed anatomy of the Australian species, Amimopina macleayi, was described by Solem (1964). Mordan (1992) postulated that Bulimus subangulatus Pfeiffer from the Lao Mountains, Cambodia, was a species of Amimopina, based on the two shells in the syntype series in the Natural History Museum, London. Since then, Sutcharit et al. (2010) have recorded subangulatus from Thailand, along with two species of Rhachistia, and have shown subangulatus to be an Amimopina on the basis of dissection. We are thus gaining ever more detailed information on the continental distribution of the Cerastidae in the Australasian region, and this is turning out to be far less disjunct than previously thought.