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 Protobranchiata: 



CK Hinge with teeth 1. NUCULIDAE 



a, 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: 



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



sinus 1. NUCULA 



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



&! Pallial sinus large 2. YOLDIA 



&, Pallial sinus small 3. LEDA 



1. NTTCULA Lamarck. Shell oval, somewhat 

 triangular, with a short posterior side; periostra- 

 cum olive; foot used for burrowing: 70 species, 

 Fig. 891 Fig. 892 cosmopolitan. 

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



proximo, (Verrill). J ' V 



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



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



of teeth forming nearly a right angle; length 10 

 mm.; height 9 mm.; width 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 Holier. Shell elongate, compressed, smooth, 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. limatulaf (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 Ser., 1901. 



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

 Vol. 4, 18S9. 



