568 MOLLUSCA 



by a fringed margin ; gonads open into the kidneys ; 2 adductor muscles ; 



mantle either open below or closed; siphons either absent or present: 2 



families, both marine. 



Key to the families of Protohranchiata: 



Oi Hinge with teeth 1. Nuculidae 



c. Hinge toothless 2. Solemyidae 



Family 1. NUCULIDAE. 



Shell equivalve, oval or triangular, pearly within; hinge with great 

 numbers of saw-like teeth, interrupted by a central pit for the ligament, 

 which is either external or internal; mantle open below; no byssus; oral 

 palps very large, with a posterior appendage: about 40 species on the 

 Atlantic coast, mostly in deep water. 



Key to the genera of Nuculidae here described: 



Oi No siphons ; hinder part of the shell not prolonged and without pallial 



sinus 1. NucuLA 



a. Siphons and pallial sinus present ; hinder part of shell prolonged. 



5i Pallial sinus large 2. Yoldia 



&a Pallial sinus small 3. Leda 



1. NuCTJLA Lamarck. Shell oval, somewhat 



/^^ triangular, with a short posterior side ; periostra- 



v^^ cum olive; foot used for burrowing: 70 species, 



Fig. 89i Fig. 892 cosmopolitan. 



Fig. 891— Nucuia N. proxima (Say) (Fig. 891). Shell strongly 



proxima (Verrill). . x xi i " 



Fig. 892 — Nucuia oblique, thick; umbo prominent; hinge teeth large, 



delphinodonta (Ver- . 



rill). 12 behind and 18 before the umbo, the two series 



of teeth forming nearly a right angle; length 10 

 mm.; height 9 mm.; w4dth 6 mm.: South Carolina to Gulf of St. Law- 

 rence; often common in shallow water. 



N. delphinodonta* Mighels (Fig. 892). Shell ovate, somewhat 

 oblique; hinge teeth, 3 behind and 7 before the umbo; length 3.2 mm., 

 height 2.7 mm.; width 2.2 mm.: New Jersey to Greenland, in 6 to 100 

 fathoms; often abundant; Europe. / 



2. Yoldia MoUer. Shell elongate, compressed, smootli, and shining, 

 attenuated and gaping behind, dark olive in color; mantle open, with 

 elongated, united, retractile siphons ; animal very active, leaping 

 through the water: about 12 species on the Atlantic coast. 



y. limatulat (Say) (Fig. 893). Shell 48 mm. long, 23 mm. high, 

 and 13 mm. wide; umbo near the center, the posterior dorsal slope 



* See "Life History of Nucuia delphinodonta," by G. A. Drew, Quart. J. M. S., 

 Vol. 44, New Scr., 1901. 



t See "Yoldia limatula," by G. A. Drew, Mem. Biol. Lab., .Tobns Hopkins, 

 Vol. 4, 1889, 



