known as Gemma, Parastarte, and Psephidia. 177 



and the subgenus. It may be added that only one living 

 species of Gemma (with two well-marked varieties) and one 

 species of Parastarte are known, and both are inhabitants of 

 the Atlantic coast of North America ; but two other species 

 of Gemma have been recorded from the Tertiary deposits of 

 Florida. The following is an amended description of this 

 little group of shells. 



Genus Gemma. 



Shell small, oval or snb trigonal, smooth or concentrically 

 striated. Lunule large, superficial, circumscribed. Escutcheon 

 not defined. Hinge-plate short ; the right valve with three 

 divi rgent teetli, the median large and triangular, the other 

 two narrow and inconspicuous; tho left valve with two or 

 three teeth, the median or its representative- being always 

 bifid. Antero-dorsal margin of the left valve and postero- 

 dorsal margin of the right valve grooved to receive the 

 opposite ridged margins. Ventral margins crenulated. 



Gemma, s. s. — Type, Venus gemma, i'otten. 



Shell short, striate, white or purplish. Three cardinal 

 teeth in each valve. Marginal ridges distinct and the grooves 

 deep. Pallial sinus varying both in shape and size, being 

 sometimes short and angular, sometimes deep, ascending, 

 and rounded. 



Subgenus Parastarte, Conrad. 



Type, Astarie triquetra, Conr. 



Shell taller than broad, strong, equilateral and subtrigomd, 

 smooth, yellowish brown in colour. Hinge with three teeth 

 in the right valve and two in the left. Ligament very short. 

 Marginal ridges and grooves narrow. Pallial line only 

 slightly inflected. 



3. PsErniDiA, Dall. 



A little group of shells was named Psephu by Carpenter in 

 1864, and was more clearly defined from the type of P. lordly 

 Baird, in lbGa, but was re-named Psephidia by Dr. Dall in 

 1902 * because he found that the name Psephts had been 

 used in Lepidoptera by Guene'e in 185-1. The original 

 Psephi s of Carpenter included several species, but Dr. Dall 

 found that L\ (autilla had an anterior lateral tooth and 

 belonged to his Trajisenella^ also that the supposed P. telli- 

 myalis and P. salmonea were the uepiouic young of other 



* Proc. U.S. Nat. Mas. vol. xwi. p. •Jiio'. 



