84 CARDIUM.— DONAX.— CYCLAS.— CORBIS.— TELLINA. 



2. In this family we place Cardium, Donax, Cyclas, Corbis, 

 Tellina, Lucina, Venus, Petricola, Mactra, &c., genera which, 

 for the most part, are very numerous in species. 



3. The C AUDI A— Car divm — are chiefly characterized by 

 the hin^e, which consists of two small teeth in each valve, situate 

 in the centre, and a projecting plate or tooth placed at some 

 distance in front and behind. The shell is equivalve, convex, 

 and furnished with ribs radiating from the summit towards 

 the circumference. And when looked at from one side, its 

 general form reminds us of that of a heart, from the manner in 

 which the summits (beaks) of the valves rise up, and curve in- 

 wards towards each other. 



4. The DoNACEs — Donax — have a hinge formed nearly like 

 that of the Cardia; but their shell is flattened, nearly triangular, 

 inequilateral, and the summits are nearly vertical. Several small 

 species are found on the coasts of France, the shells of which are 

 very pretty. 



5. The Cyclades — Cyclas — have the same hinge and are 

 distinguished from the preceding by the rounded form of the 

 shell, which is equilateral (that is, the two halves of the valve, 

 situate on each side of a vertical line drawn through its summit, 

 are alike), and striated transversely. One species, Tellina 

 cornea, is common on the coast of France. The genera Cyrena 

 and Cyprina belong to this group. 



6. The Corbis are transversely oblong marine shells, which 

 resemble the preceding in the hinge; their external surface is 

 very regularly marked by transverse ribs, crossed by diverging 

 rays. 



7. The Telling {fg. 105) — Tellina — closely resemble the 

 Donaces; but they have the middle of the hinge armed with one 

 tooth to the left, and two to the right. The shell is slightly 

 gaping. Like the Donaces, they have two long tubes (Jig. 105), 

 which can be entirely concealed within the shell when the animal 

 contracts. Both live buried in the sand and are found on the 

 coast of France. 



2. What are the principal genera belonging to the family of Cardiacea ? 



3. What are the characters of the genus Cardium ? 



4. What are the characters of the genus Donax ? 



5. How is the genus Cy'clas characterized ? (Cyclas, from the Greek, 

 tuklos, a circle.) 



6. What are the characters of the genus Corbis ? 



7. How is the genus Telli'na characterized ? 



