216 TRANS. OF THE ACAD. OF SCIENCE. 
/ 
numerous towards the borders and on the ears, and — 
spring from the — ; visceral region for a short distan 
from the beak marked with irregular, concentric, waving, m 
or less, prominent is ventral valve strongly arched, slightly 
flattened on the visceral region and towards the anterior 
margin; ornamented with cost and ruge like the opposite 
valve; mesial ridge corresponding to the dorsal sinus. Jnlernal 
surface of ventral valve garnished with a prominent, trifid 
cardinal process, fortified at its base with three diverging ioe 
two extend laterally nearly parallel to the cardinal line, 
become obsolete on the-ears; the third or the mesial oe 
> . and in 
former ridges are the oval, rugose scars of the a dductor muscles. 
Vascular ar impressions ovate, a, gt ok anterior and lateral bor- 
ders, connected by recurved sinuses to the anterior part | 
the mesial ridge; central portion of the visceral region pune- 
tate, and marked with longitudinal coste ; around the anterior 
border is a zone ornamented with tubes; ; those on the inner 
pe prominent, while those nearer the border 
“ and more numerous. — of dorsal 
re small, de ceed. 
raed with oblong, elliptical, eget ductor 
wn “Thee | *, 1.653 3 Dreadth, 2255 
| Var. — aes bie Swallow —Thig shel difir ‘frou tle 
| ° Bpreiet 
— 
With cone 
alhor auc closely allied ‘to th the P. seni 
var. antique . K. (An, Fo) ee 
= it it in having the cost 
rply carinated, and 
