218 TRANS. OF THE ACAD. OF SCIENCE. 
ed, slightly flattened on the visceral region, and towards the 
anterior border marked with broad, depressed, irregular longi- 
tudinal cost; of these about four on each side of the mesial 
sinus are much larger and extend from the visceral region 
to the anterior border; the whole surface ornamented with 
large, OT spines, arranged somewhat regularly in diagonal 
lines; mesial sinus deep and broad; ventral valve strongly 
arched, slightly flattened on the visceral region and towards 
he anterior margin; marked with longitudinal depressions 
nding to the costz of the opposite valve; mesial ridge 
conspicuous on the anterior prolongation, the whole marked 
with deep pits about as numerous as the spines of the oppo- 
site valve; visceral — s of both valves have indistinet, ir- 
‘am transverse 
inus is not so large, the costie ema age 
—. mig ore tubes on nape dorsal - valee a ears, 
and and pits on the ventral valve, larger and more ere 
