256 LECTURES ON MOLLUSCA. 



Vanganella has the shape and internal rib of Machcera, with a very 

 projecting cartilage-pit, lying against the rib. 



Family Veneridye. 



The Venus-tribe may be regarded as the types of the Lamellibranchs, 

 presenting the greatest balance of characters. The animals have 

 rather short pipes, fringed at the ends, and more or less united ; the 

 incurrent being the longer of the two, contrary to the usual habit. 

 The mantle is closed in front, with a large opening for the tongue- 

 shaped foot. They are found in all seas, generally in shallow water. 

 They first appear in the oolitic strata, and are now at their maximum 

 of development. The shells are strong, almost devoid of structure, 

 very beautifully colored and sculptured, and held together by a stout, 

 external ligament. The hinge teeth are very large, and generally 

 divergent. As among the snails and other large families, there are so 

 many intermediate forms between the extremes that the division into 

 genera is a matter of great difficulty. The most elaborate classification 

 of the species is to be found in Deshayes' British Museum Catalogue. 



The shells of Trigona somewhat resemble Erycina and the Mactrids. 

 They are triangular, with from three to six hinge teeth, and one rather 

 long side tooth. The tertiary fossil Gratelupia greatly resembles it, 

 with an additional number of small parallel posterior teeth. Meroe is 

 wedge-shaped, with the margin crenulated, and the ligament in a 

 deep-cut groove. Cytherea has a heavy shell, with a tooth next the 

 ligament crenulated, and the outside tooth transverse. The mantle- 

 bend is very slight. Callisla, (which is the Dione of the British Mu- 

 seum Catalogue, and includes most of the species grouped together as 

 Cytherea by Lamarck,) has a wide mantle-bend, the pipes being rather 

 long, and united as in Mactra. The hinge teeth are 3-4, the outer 

 being short, but transverse. Dosinia also has united siphons, with an 

 angular mantle-bend. The shells are somewhat twisted spirally, with 

 close concentric furrows, and a sharply-cut lunule. In Cyclina, the 

 shell is thin, inflated, and without lunule, resembling Lucinopsis ; but 

 the animal closely resembles Dosinia. dementia has a very thin 

 shell, with a hinge resembling Venus, but pipes and mantle-bend 

 like Dosinia. 



The restricted genus Venus has the pipes separate and diverging ; 

 with a short angular mantle bend. The hinge-teeth are 3-3, nearly 

 equal and spreading. The valve margins in this group are crenulated, 

 corresponding with the fringing of the mantle. In Chione, (a bad 

 name, because it does not include the old Venus chione, now a Callista,) 

 the pipes are short and united at the base. The mantle-bend is very 

 slight ; and the teeth are 3-2, one being longer than the rest. The 

 common Mercenaria, or u clam" of the Atlantic States, has the area 

 inside the ligament coarsely furrowed. Anomalocardia has irregular, 

 thick, triangular shells, with two teeth in each valve, and the mantle- 

 bend almost obsolete. The little New England Gemma has the hinge 

 of a Venus, the external aspect of a Circe, and the deep angular mantle- 

 bend of a Dosinia. 



The Tapes group have oblong, transverse shells ; with narrow, com- 



