CRASSATELLA. 



Plate I. 



Genus Crass atella, Lamarck. 

 Testa aquivdlvis, transversa, iniequi/atcralis.undique clausa. 

 Curdo in valvd altera dentibus duobus ; in altera dente 

 unico ; foveold in utrdque valvd antice adjeetd, liga- 

 mei/to profundi inserto. Dentes laterales nulli, aut 

 'obsoleti. Impressiones musculares distinct issiml im- 

 butie. 

 Shell equivalve, transverse, inequilateral, and everywhere 

 closely shut. Two cardinal teeth in one valve and 

 one in the other, with a deep pit on the anterior side 

 in each for the reception of the ligament. Lateral 

 teeth obsolete, or wanting. Impressions of the two 

 lateral muscles very distinctly marked. 

 The genus Crassatella was instituted by Lamarck for 

 the purpose of associating a group of acephalous mol- 

 lusks that appear to have been quite unknown to Lin- 

 naaus. Their shells exhibit an interesting peculiarity of 

 character, being of a thick and solid nature, with an in- 

 ternal ligament, and for the most part covered with a 

 strong brown epidermis. The genus, however, as intro- 

 duced by this distinguished conchologist, was yet imper- 

 fect; it included five species that could not easily be 

 separated from his Amphidesmata, and was therefore 

 susceptible of improvement. With the view of uniting 

 the intermediate species of these genera, a new genus 

 was very judiciously proposed by Deshayes, under the 

 title of Mesodesma ; and, as it thus includes a series which 

 are not strictly referable to either, it is now commonly 



adopted. 



Out of eleven species referred to this genus by La- 

 marck, six only can be allowed to remain. Since his 

 time, however, we have had many important additions ; 

 two new species were described by Sowerby, in the ' Pro- 

 ceedings of the Zoological Society' for 1832 ; one by the 

 same author, in his ' Appendix to the Tankerville Cata- 

 logue ;' and ten by myself, in the ' Proceedings of the 

 Zoological Society' for the past year. 



Of the following nineteen species, eleven have never 

 been figured before. 



Species 1, a and b. (Mus. Cuming.) 

 Crassatella gibbosa. Crass, testd subovali, gibbosa, 

 epidermide nigerrimo-fuscd indutd ; intiis albicante, 

 latere postico brunneo ; umbonibus undulatis, com- 

 pressis ; latere antico inclinato, rotundato, sulcis non- 

 nullis brevibus; postico arcuato, subroslrato. 



The swollen Crassatella. Shell nearly oval, gib- 

 bous, and covered with a very dark brown epider- 

 mis ; interior whitish, brown at the posterior end ; 

 umbones waved and peculiarly compressed ; ante- 

 rior side bent, rounded, and ornamented with a few 

 short grooves ; posterior side curved, and somewhat 

 beaked. 

 Sowerby, Proceedings Zool. Soc, 1832, p. 56. 

 Rekve, Conch. Syst., vol. i. pi. 44. f. 2. 

 Hub. Western Coast of South America. 



Several specimens of this interesting species were 

 dredged up by Mr. Cuming from sandy mud in eleven 

 fathoms water, off St. Elena and Xipixapi, in South 

 America. The specimens here figured are the oldest 

 and the youngest which Mr. Cuming collected, and 

 are admirably adapted to show the different appearances 

 this shell exhibits at different periods of growth. At an 

 early stage the valves are depressed and strongly undu- 

 lated ; they then become gradually gibbous, and the un- 

 dulations, retaining their original formation, get pressed 

 up to the umbones. 



Species 2, a and b. (Mus. Cuming.) 



Crassatella uxddlata. Crass, testd ovali, brunned.fus- 

 co-maculatd, epidermide rubido-fuscd indutd ; intiis 

 brunned, prope marginem anticam albicante; umbo- 

 nibus undulatis; latere antico inclinuto, rotundato; 

 postico longiore, angulifero. 

 The waved Crassatella. Shell oval, brown, faintly 

 spotted, and covered with a reddish brown epi- 

 dermis; interior bright transparent reddish brown, 

 white near the anterior margin ; umbones strongly 

 undulated ; anterior side bent and rounded ; poste- 

 rior side longer, and angulated. 

 Sowerby, Proceedings Zool. Soc, 1S32, p. 56. 

 Hab. Puerto Portrero, Central America. 



Tins species was dredged by Mr. Cuming off the above- 

 mentioned coast, from sandy mud in eleven fathoms water. 

 The observations that were made on the different appear- 

 ances of the Crassatella gibbosa, at different periods of 

 growth, apply also to the Crassatella undulata ; the un- 

 dulations are, however, wider and much more strongly 

 developed. The epidermis of this shell is of a redcli-h 

 brown colour, and (in the adult) of very thick fibrous 

 texture. 



Jan 1343. 



