276 



LOWER INVERTEBRATES. 



In Cyclocardia the shell is radiately ribbed, and nearly circular in outline. Cardinal, 

 but no lateral teeth are present. Isocardia is a genus with very ventricose shell, and 

 with the beaks regularly inrolled. Isocardia cor is found in the Mediterranean. 



With the Venerid^ we enter upon a group embracing all the remaining bivalves, 

 in which the siphons are well developed and the pallial sinus well marked. In the 

 Veneridffi the shell varies in outline between nearly spherical to oblong, the ligament 

 is external, and usually there are three diverging teeth in each valve. The siphonal 

 tubes are unequally developed, and are united at the base. The foot is tongue-shaped, 

 and compressed, and the triangular labial palpi are very large. The forms embraced 

 in this family include some of the handsomest of the bivalve molluscs, the distribution 

 of color being frequently very striking, chevron-shaped lines being most frequent. 



In Ventts, the typical genus, the shell is oval and thick, and the pallial sinus is 

 small and angular. Most important to us is the quahog, round clam, or hard-shell 

 clam ( Ve?ius mercenaria), which forms a considerable article of diet in those regions 

 where the long clam, or soft-shell clam (Mj/a arenaria), is not to be had. In its 

 range it extends from Texas to Cape Cod, but north of that cape it is comparatively 

 rare and local. It is " common on sandy shores, living chiefly on the sandy and muddy 

 flats, just beyond low-water mark, but is often found on the portion laid bare at low- 

 water of spring tides. It also inhabits the estuaries, where it most abounds. It bur- 

 rows a short distance below the surface, but is frequently found crawling at the surface, 



with the shell partly 

 exposed." The mantle 

 is widely open in front, 

 allowing the large foot 

 to be placed in almost 

 any position. The si- 

 phonal tubes are united 

 for quite a distance. 

 "This species is taken 

 in large quantities for 

 food, and may almost 

 always be seen of vari- 

 ous sizes in our markets. 

 The small or moderate- 

 sized ones are generally 

 preferred to the full- 

 grown clams. Most of 

 those sold come from the muddy estuaries, in shallow water, and are fished up chiefly 

 by means of long rakes and tongs, such as are often used for obtaining oysters. Some- 

 times they are dredged, and occasionally they can be obtained by hand at or just below 

 low-water mark. These estuary specimens usually have rough, thick, dull-white or 

 mud-stained shells, but those from the sandy shores outside have thinner and more 

 delicate shells, often with high, thin ribs, especially when young ; and, in some varieties, 

 the shell is handsomely marked with angular or zigzag lines or streaks of red oi 

 brown." 



In most of the shells, when nearly full-grown, the borders of the interior of the 

 valves is colored purple to the distance of about half an inch from the margin, and it 

 was by breaking up this portion and converting it into beads that the Indians of New 



Fig. 296, — Fismfs mercenaria, quahog, with foot, siphons, and edge of mantle 

 extended. 



