404 



MOLLUSCS, 



without a pallial sinus, and pearly within. The hinge is composed of numerous 

 pointed interlocking teeth on each side of the cartilage-pit beneath the umbones. 

 Unlike Nucula, the genus Nuculina 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 a few representatives 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 remarkable 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 chestnut-coloured horny periostracum, which, when dry, is very 

 brittle. Only about six species of one genus, Solenomya, are know r n, 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 FILIBRANCHIATA. 



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 Anomiidce 

 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 

 mollusc attaches itself to rocks and stones. 

 The animal has a small foot ; the mantle is 

 free all round, and there is but a single 

 central adductor muscle (m). About forty 

 species are known, two of which occur in 

 Britain. Placuna is another genus of this 

 family, in which the shells are very flat, with- 

 out any byssal opening ; the valves being thin, 

 somewhat nacreous, with two long divergent 



hinge-teeth to which the ligament is attached. About half a dozen species from 

 the Indo-Pacific Ocean are known. P. sella has a somewhat wavy or cockled 

 appearance, and is known as the saddle-oyster, on account of its saddle-like form. 

 The arks (Arcidw) are nearly all strong heavy shells, generally equivalve, but in 



RIGHT SIDE OF Anomia, WITH SHELL REMOVED. 



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

 Calcified byssus. 



