A NEW PISIDIVM MOLLUSK—STERKI. 



477 



laminae; cardinal teeth rather small, the right slightly curved to 

 nearly straight, with its posterior end equally strong or slightly 

 thicker ; left anterior near- 



ly straight, the posterior 

 curved, oblique; lammae 

 rather stout, their cusps 

 ("lateral teeth") some- 

 what proximal, the left 

 anterior short, abrupt, the 

 others less abrupt, pomt- 

 ed ; outer ones in the right 

 valve small, especially so 



Fig. 1. — PisiDiuM hannai. Lateral view. 



Length, 



the posterior; ligament and resilium short, rather thick. 

 3.4; height, 3; diameter, 2.2 mm. Soft parts not seen. 

 Type.— C&t. No. 273767, U.S.N.M. 



Habitat. — St. Paul Island, of the Pribilof Islands, Behring Sea, 

 collected by Mr. G. D. Hanna, in whose honor the species has been 



named. A good number of specimens 

 was secured, from several places, partly 

 associated with P. scutellatum Sterki. 



P. hannai does not closely resemble 

 any of the known North American spe- 

 cies, and can not be mistaken for any 

 of them. Its distinguishing features 

 are: Its size, the elliptical outhnes in 

 lateral aspect, with the supero-anterior 

 slope not marked, the broad beaks, 

 and the liinge-plate being broadest in 

 the middle. 



Tlie species is rather variable in 

 several respects. Some specimens are 

 more inflated above than shown in the 

 figure, and from there the valves show 

 nearly a straight Hue to the ventral 

 margin, in frontal (or rear) aspect; an 

 upper line of growth is often rather deep and marks off the beaks 

 like the "cap" in Musculium, probably due to the long winter sea- 

 son. In some instances the demarcation is so abrupt as to form a 

 sharp ridge for rib ; in some specimens the hinge is slighter, yet with 

 the plates broader in the middle. It may be noted that in one 

 specimen the posterior part of the hinge is reversed — that is, the 

 posterior laminae. 



Fig. 2.— Pisidium hannai. End vtew and 



HINGES. A, anterior; P, POSTERIOR; 

 L. v., LEFT valve; R. V., RIGHT VALVE. 



