364 PROCEEDINGS OF THE NATIONAL MUSEUM. 



Temperate and tropical regions of the Old World. This is Amyg- 

 dala Romer, 1857, not of Van Phelsum, 1774; CJuneus H, and A. 

 Adams, 1857, but not of Da Costa, 1776. It is not Amygdalwn Megerle, 



1811. 



Section Rad'dape^ s.s. (See above.) 



Subgenus Protothaca Dall, 1902. Type, Venus thaca Molina (+ ^' . 

 d<ynd)eyi Lanuiix'k). 



Shell ovate, convex, coloration white or dull; surface dull, reticu- 

 lately sculptured, the radials usually stronger; sculpture more or less 

 distinctly divided into three areas, the middle of the valves with 

 chiefly radial, the anterior radial and scabrous, the posterior with 

 irregularly concentric sculpture; lunule and escutcheon of the left 

 valve, sharply circuniscribed; in the type species the right valve shows 

 no escutcheon and the margin partiall}^ overlaps that of the left valve 

 but does not conceal the ligament; middle cardinals grooved or ]>itid; 

 pallial sinus free, moderate, pointed in front; the inner margins 

 sharply crenulated in the typical section. 



Section Protothaca s. s. (See above.) The siphons are short and 

 united, the foot hatchet-shaped and not bj^ssiferous. The distribution 

 of this group includes the west coast of America, fJapan, and New 

 Zealand ( Y. costata Quoy). 



Section CalUthaca Dall, 1902. Type, Tapes tenerrlma Carpenter. 



Sculpture delicate, uniform over the disk and reticulate except in 

 distorted individuals; lunule feebly defined with no escutcheon; the 

 dorsal margin not overlapping in the right valve; inner margins entire, 

 otherwise as in Protothaca. 



Distribution, Northwest America. The tropical species of Proto- 

 thaca are maculated, the northern forms yellowish white, with a dull 

 surface. There is no byssal groove and the papillose siphons are 

 united to their tips in the type species. The group is Saxiaomiis /? 

 of Desha3^es, 1858. 



Genus LIOCYMA Dall, 1870. 



Type, Venus fluctaosa. Gould. 



Shell small, white or unicolored, covered with a vernicose perios- 

 tracum, and concentrically waved, without radial sculpture; lunule 

 circumscriV)ed, escutcheon absent; inner margins smooth; pallial sinus 

 short, free, rounded triangular; three cardinals in each valve, the 

 anterior right and posterior right, entire, the others bitid; siphonal 

 tubes unequal, the anal shorter, l)oth united to their tips; foot long 

 and pointed, without a byssal groove; the mantle open ventrally and 

 smooth edged. 



Distribution, Boreal and arctic waters of the Northern Hemisphere. 



The group was called Lyocruia l)y Barrois in 1887, and the species 

 were formerly referred to Tapes. 



