44 TRANS. OF THE ACAD. OF SCIENCE. 
Ventral valve sabquadrate, irregularly concave following the 
concavity of the dorsal valve, anterior border much thickened 
the longitudinal ribs radiate from a perforated point nearly oppo- » 
site the umbo, and are crossed by irregularly branching, coarse, 
concentric ridges, which give to this surface a beautiful reticulate 
aspect; it is further marked by a slight, broad and trian 
relet, slightly depressed in the m iddle by a very daha sinus, 
which is also triangular in form, widening much as it procee 
from the perforated point to the anterior border. This sinus and 
the sa amcecane which occur between the outer borders of the bou- 
relet and inal angles give place to three depressions on — 
this surface of she valve; the bourelet with its middle sinus occu- 
pies nearly the whole of the viseeral disk. Spine holes are found 
here and there in the depressions between the longitudinal ribs and 
the concentric ridges, arranged mony alternately, or quidcuncial- — 
ly, as upon the doveck adie a little beyond the perforated point, 
i 
Z 
giving rise to two points, which correspond to 
similar points on the hinge line of the dorsal valve. The structurt 
. of the: articulation of the valves, apart from the hinge lines, was 
nsions a gence reting on Pad lng : 
nearly erect. In Fig. 14, fo nop weet 
erect. = our 
tube for the mes of the pits Sp nF twee 
™ & 
‘ 
the hinge line i is thickened into a small tubercle, bisected by a trie 7 
