LECTURES ON MOLLUSCA. 251 



Family ANATiNiDiE. {Lantern- Shells.) 



The shells in this family care almost always thin, pearly within, and 

 roughened outside. They have an internal cartilage, supported on a 

 spoon-shaped plate at the hinge, and strengthened by a shelly 

 "ossicle" within. Anatina has the spoon supported by a clavicle at 

 the umbos. The oolitic fossils, Cercomya, have the valves concentri- 

 cally furrowed. In the nestling Tyleria, (of which only one specimen 

 is known from Mazatlan, the clavicle is loose, twisted round the side 

 of the shell, and united to it by numerous bridges. Periploma has a 

 rectangular shaped body, without clavicle. Lyonsia has a shell of 

 irregular growth, like Saxicava; and a very small spoon close to the 

 umbo. Its Californian neighbor, Mytilimeria, lives imbedded in the 

 nests of Tunicaries, and can scarcely open either its valves or its 

 mantle. The beaks of the shell are spirally twisted., as in Isocardia. 

 The shells of Thracia are not pearly, and are very rough outside. 

 Some of the species are nestlers and distorted like Lyonsia. 



The very beautiful shells of Necera are shaped like a Corbula, with 

 produced beak to shelter the delicately fringed pipes. They are thin 

 and pearly, and only found in deep water. Theora lives in shallower 

 water, is more compressed, and has a very wide mantle-bend like the 

 Tellens. Thetis has very short siphons, and a very long tubular foot; 

 the hinge resembles the Kelliads. 



Two singular groups are placed here provisionally, until the animals 

 have been examined. The African Tugonia (also found fossil in the 

 Pliocene) has a globular, twisted shell, somewhat resembling Necera, 

 with a very large spoon-shaped cartilage-pit, and a very small mantle- 

 bend. Anatinella is shaped somewhat like Myodora; with very long, 

 narrow cartilage pits, and no bend in the mantle line. In this respect 

 it resembles many of the Corbulids. 



Family Pholadomyims. 



There is only one living representative (from the West Indies) of a 

 large tribe of puzzling fossils, which have received various names with- 

 out much being known of their affinities. The living animal agrees 

 with Anatinids in having only one gill on each side, but differs from 

 all its predecessors in the mantle having a fourth opening in front. 

 The ligament is external. The principal fossil forms which used to 

 be classed under the general names of Pholadomya and Amphidesma, 

 Elnio, &c, have been separated as Homomya, with thick shell and 

 concentric sculpture ; Myacites with Goniomya, Tellinomya, Grammysia 

 and Sedgeivichia; Ceromya, Gresslya, Cardiomorpha and Edmondia. 



Family Myochamid/e. 



This is a small group of attached shells, representing as it were the 

 oysters and Chamas among the Anatinids. The animals have strong 

 points of resemblance with Pholadomya, having a minute ventral 

 opening. The ligament is internal, and has an ossicle as in Anatina. 

 Myochama lives on other shells at great depths, and has a small mantle- 



