250 LECTURES ON MOLLUSOA. 



by a lyssus, which they spin by their small grooved foot. It is said 

 that five genera (placed in different families) and fifteen species have 

 been made out of different conditions of the Saxicava arctica, which 

 has spread itself over the northern hemisphere from the time of the 

 middle tertiaries, having attained its greatest development in the drift 

 period. The Cyrtodaria of Newfoundland is one of the coarsest of 

 shells, covered with a horny skin, which in drying often cracks the 

 shell inside. Glycimeris has a shell exactly like Panopwa; but the 

 animal is a gigantic Saxicavid. The long pipes are united almost to 

 their ends, the gills protruding into them; and the mantle-line in the 

 shell is broken into joints. The shells gape all round like Pholas, but 

 have a strong external ligament fixed to stout fulcrum s. 



Family Myidje. {Gapers.) 



In the Myas (called "Clams" in New England, and brought to 

 market for food,) the shell is tolerably regular } and covered with a 

 wrinkled skin which is produced over the pipes. These are united, and 

 fringed at the end. The species are widely diffused, in time and s})ace, 

 and are generally pretty large. The cartilage is fixed in a pit between 

 a projecting spoon-shaped tooth in the larger valve, and a hollow in the 

 smaller. The Californian Platyodon has the pipe-ends strengthened 

 by four shelly valves, reminding us of Teredo. The name My a was 

 given by Linnaeus to all shells with an internal cartilage ; but the 

 character is not always constant in the same family. Panapcea (to 

 whicli and to Pholadomya most of the fossils called " My a" belong) 

 has an external ligament, and small interlocking hinge-teeth, like 

 Glycimeris. Lutraria has a shell resembling the New England 

 "clam," but of more porcellanous texture; and with a spoon-shaped 

 process in each valve to support the cartilage by the side of a tooth. 

 Several shells generally associated with it by American authors have a 

 Mactroid animal. The great Californian Tresus, which is eaten at 

 Puget Sound, has small teeth on each side of the cartilage pit. Seiz- 

 ocheilus may prove to be identical with Tresus; it has two horny valves 

 at the end to protect the pipes. The animal of Fastonia has not been 

 examined ; but the shell is like a heavy, swollen Latraria, with radi- 

 ating furrows outside. 



Family Corbulid^;. (Basket- Shells.) 



The Corbida group are like little Myas, but they scarcely gape, and 

 have very short pipes, fringed at the ends. The foot is finger-like, 

 adapted to poke in mud and sand, where they live often in immense 

 profusion. They have one valve much smaller than the other; the 

 hinge consisting of a conical tooth by the side of a cartilage pit in each 

 valve. Potamomya includes the flattened estuary species ; and Corbu- 

 lomya some of the fossil forms, which begin to appear in the oolites. 

 Sphcenia has the nestling habits of Saxicava, with the front end of the 

 shell very short. Cryptomya has a Myoid hinge, with a shell inter- 

 mediate between that and Sphwnia. 



