LECTURES ON MOLLUSCA. Ill 



the Venerids, the Cyprinids, and the Cockles. The animals however 

 differ (so far as yet known) in having no true breathing pipes, but 

 only a fringed opening in the mantle, as in the Unios. The foot is 

 tongue-shaped, and the creatures are of sedentary habits, sometimes 

 burrowing in coral. They form one of the most extensive groups of 

 bivalves in the secondary and older tertiary strata ; but now most of 

 the forms are extinct, and others are dying out. It is probable that 

 some of the following genera really belong to the Cyprinids. 



The first division have shells furrowed like the Cockles. Veneri- 

 cardia also resembles that group in having a bent foot for leaping ; 

 but the shape and hinge more resemble Venus. Cardita has some- 

 what the shape of Rupellaria, and has a short lateral tooth within the 

 ligament. Thecalia has a curious cup inside the valves to receive the 

 eggs. Trapezium has 3-3 hinge teeth, besides the lateral. Corallio- 

 phaga is shaped like Lifhophagus , and is also a borer ; but the hinge 

 resembles Trapezium. 



The oolitic fossil Myoconclia is shaped like Modiola, but was closely 

 related to Cardita. It has a long tooth at the beaks, which is often 

 encroached upon by the hinge-margin as in old specimens of Cardita 

 orbicularis. Hippopodium (peculiar to the English Lias) has a very 

 thick, irregular, toothless shell, looking like a gigantic Saxicavid. 

 Cardinia and Anthracosia have ZTmo-shaped shells, abundant in the 

 oolitic age, with a hinge more resembling the Cockles. Pacliyrisma 

 and Opis form a passage to the Heart-cockles. Cypricardites, Pleuro- 

 phorus, Megalodon, Goldfussia, Megaloma, and Pachydomus are palae- 

 ozoic forms, the relations of which are not yet properly ascertained. 



The Astarte race are generally flattened shells with concentric sculp- 

 ture. The fossil species abound in the oolites and tertiaries; the recent 

 are few in number, covered with a thick, dull skin, and mostly from 

 the boreal and north temperate zones. In the warmer seas are found 

 small Astartoid shells with lateral teeth, called Gouldia. In the trop- 

 ical regions of the east are found a group of shells with hinge resem- 

 bling Trigona, but without mantle-bend. They are called Circe, and 

 have a peculiar flattening at the beaks. 



One group, related to the other members of this family in the ani- 

 mal, has the cartilage internal, as in Semele and Mesodesma. Crassa- 

 tella has a ponderous shell with a stout hinge and short lateral teeth. 

 It is found fossil from the cretaceous age. The shells of Davila are 

 rounded and flattened, like Felania. 



Family CHAMIDJS. 



The Chama-tribe seems to interrupt the natural sequence of the 

 families, presenting us with a race of irregular shells like oysters, 

 always attached, and generally covered with spines or ridges, like the 

 Spondyli. The shells are known by the two strong muscular impres- 

 sions, and the ^mo-shaped teeth at the hinge. The umbos are more 

 or less twisted into a spiral, as in the Heart-cockles. The animal ap- 

 pears to resemble a stationary Isocardia, with the mantle closed in 

 front, and very short pipes. The foot is bent, as in the Cockles, but 

 its use is not clear. They are found only in the warmer seas, begin- 



