3374 



THE MOLLUSKS OR SHELLFISH 



with scarcely any byssal gland. The families Nuculidte and Solenomyidce constitute 

 this order. The mollusks belonging to the former are all marine; and have the 

 mantle free all round, or forming two small posterior siphons. The gills are small, 

 but the labial palpi very large. In the typical Nucula the shell is small, more or 

 less triangular, generally covered with a greenish olive periostracum without a 

 pallial sinus, and pearly within. The hinge is composed of numerous pointed inter- 

 locking teeth on each side of the cartilage pit beneath theumbones. Unlike Nucula, 

 the genus Nucidina is provided with two small adjacent siphons. The shell is 

 usually somewhat produced or beaked posteriorly, has a slight pallial sinus, and is 

 not pearly within. The hinge teeth and the resilium are as in Nucula. Yoldia is 

 like Nuculina in shape, but has longer siphons, and the periostracum more glossy. 

 The shells of Malletia are like those of Yoldia, but the ligament is external. Nucula 

 and Nuculina have a world-wide distribution, and are numerously represented in 

 species. Yoldia and Malletia, on the contrary, have comparatively few representa- 

 tives in Arctic, Northern, and Antarctic regions. The fossil forms belonging to this 

 family are far more numerous than the recent, and include several generic groups 

 which no longer exist. In the second family, Solenomyidce, the animal is remark- 

 able for its proboscis-like foot, expanded at the end into a flattened disc with a 

 dentate edge. The mantle is united ventrally, but open in front for the passage 

 of the foot, and posteriorly for the siphons. The shell is elongate, compressedly 

 subcylindrical, without hinge teeth, and clothed with a thick dark chesnut-colored 

 horny periostracum, which, when dry, is very brittle. Only about six species of 

 one genus, Solenomya, are known, but these are widely distributed, being found in 

 the Mediterranean, on the east coast of North America, in Patagonia, the Indian 

 Ocean, Australia, and New Zealand. 



Order FIUBRANCHIATA 



In this group the gills are smooth and their parallel filaments are directed 

 ventrally, reflexed, and provided only with ciliated interfilamentary junctions; 

 the foot being usually furnished with a byssal gland. In the family A?wmiidcz 



the shells of the typical genus Anomia are 

 generally very irregular in their growth, 

 inequivalve, and somewhat pearly within; 

 the more convex valve being remarkable for 

 the large number of muscular impressions, 

 and the flat valve for a perforation near the 

 hinge. This aperture is for the passage of a 

 calcified byssus (n}, by means of which the 

 mollusk attaches itself to rocks and stones. 

 The animal has a small foot; the mantle is 

 free all round, and there is but a single 

 RIGHT SIDE OF Anomia, WITH SHELI. central adductor muscle (*). About forty 

 REMOVED. species are known, two of which occur in 



a. Opening for hinge ; m. Adductor muscle ; n. 



calcified byssus. Britain. Placuna is another genus of this 



