LECTURES ON MOLLUSCA. 257 



pressed hinge-teeth, often bifid. The animal has a long foot, grooved 

 and often furnished with a lyssus. They are rather sedentary in their 

 habits, hiding themselves in corners, and sometimes even burrowing 

 in rock like the Saxicavids. The same species are however found on 

 the same shores, either boring or free in the sand. The siphon-pipes 

 are partly separate, and beautifully fringed ; and the mantle-bend is 

 deep. They most abound in the Old World. But on the northern shores 

 of the Pacific is found a remarkable group, Saxidomus, with additional 

 and somewhat irregular teeth, (as in Trigona,) a posterior gape, and 

 no lunule. 



Family Petricolid^;. (Boring-Venus Tribe.) 



These creatures have the mantle closed in front, like the Saxicavids, 

 with an opening for the small, pointed foot : but the pipes are short 

 and partially united, as in the Venus tribe. They generally bore in 

 shells or rock; but the opening is irregular, and displays the "nose- 

 end" of the shells. Petricola has a shape generally resembling Gas- 

 trana, with coarsely moulded beaks. The teeth are 2-2, often partially 

 absorbed by the cartilage area, which in the Choristodon section is 

 somewhat internal. Rupellaria is Tapes-shaped, and is an irregular 

 nestler, like Saxicava and Cumingia : the valves are generally prettily 

 cancellated. Naranio has a rectangular shell, with divaricated sculp- 

 ture outside, and bifid teeth within. All the shells in this family have 

 a wide mantle-bend. 



Family Glaucomyid^:. (Solen-Venus Tribe.) 



The shells of Glaucomya are covered with a dark green skin, and are 

 found in East Indian rivers, especially at the mouths. The hinge- 

 teeth are small, as in the Tellens, and the shape is like a very trans- 

 verse Petricola. There is a deep narrow mantle-bend, caused by the 

 retraction of the very long, united pipes. The mantle is closed in 

 front, except for the large mud-boring foot. The lips are large and 

 sickle-shaped. Tany siphon has long pipes, united nearly to the end. 



In the remainder of the bivalves, (with a few abnormal exceptions,) 

 there is no bend in the mantle-line, showing that the breathing pipes 

 are not long and retractile. This however is not a character of ordinal 

 importance. In the Venus tribe, we see the bend becoming smaller 

 and smaller, till the passage from Anomalocardia to Circe, whieh has 

 none, is scarcely sufficient for family distinction. In the following 

 families, we sometimes find two perfect but short pipes, sometimes only 

 one, sometimes a simple opening in the mantle. The mantle itself is 

 either partially or wholly closed in front ; or it is freely open for the 

 passage of the water into the gill-cavity. 



Family Cyprinidje. 



The shells of this group abound fossil from the secondary age, but 

 very few are now living. The only living Cyprina has a shell like a 

 swollen Callista, with a distant side tooth at the back. The little 

 northern shell called Circe minima has an animal like Cyprina, with 



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