FAMILY 5. MACTRACEA. 63 



ligamento profunde inserto. Dentes laterales nulli, aut obsoleti. 

 Impressiones musculares distinctissime imbutse. 



The shell of Crassatella bears a certain resemblance to that of Liitraria, 

 both in the position of the ligament, and in the absence of lateral teeth, 

 but may be easily recognised by the thickness of the valves, and their 

 rich dark brown epidermis. 



It may be described as being equivalve, transverse, inequilateral, and 

 everywhere closely shut. There are two cardinal teeth in one valve and 

 one in the other, and at the anterior side of them is situated a deep pit 

 for the reception of the ligament. There are no lateral teeth, or they are 

 obsolete. The points of muscular attachment are very distinctly marked. 



Examples. 

 PL XLIV. Fig. 1. 



Crassatella radiata, Sowerby. Tankerville Catalogue, App., p. 2. 



f. 2. 



PI. XLIV. Fig. 2. 



Crassatella gibbosa, Sowerby, Proceedings Zool. Soc. 1632, p. 56. 



PI. XLIV. Fig. 3. 



Crassatella Kingicola, Lamarck, Anim. sans vert., new edit., vol. vi. 

 p. 109. Sowerby, Genera of Shells, No. 3. 



MESODESMA, Deshayes. 



Testa ovata, transversa vel subtriangularis, subcompressa, sequivalvis, 

 crassa, plerumque clausa. Cardo dentibus lateralibus duobus, eras- 

 sis, in utraque valva ; cum foveola profunda, cochleariformi, inter- 

 jacente, ligamento inserto. Impressiones musculares inaequales, 

 anteriori majori, elongata ; impressio pallii sinu posteriori. 



