1895.] NEW YORK ACADEMY OF SCIENCES. 1 
area of the shell also continue to be faintly visible on the outer 
portion of the shell. 
This species, then, is marked by four stages of growth and 
development, of which the most prominent features are the 
following :— 
(1.) Hmbryonic.—Formation of the embryonic shell. 
(2.) Larval or nepionic.—Lengthening of the hinge line and 
acquisition of mantle-margins. 
(3.) Adolescent or nealogic.—Fixation of the hinge line, 
otherwise as the last, except that the radular ornament becomes 
irregular. 
(4.) Adult or ephebolic.—Absence of radular ornamentation 
on the valve, and great expansion of the mantle margin.” 
OBOLUS PRISTINUS, n. sp. Pl. iv., figs. lato le. 
-Shell substance calcareous (7). Valves orbicular, slightly 
straightened along the anterior border, and having a flattened, 
flange like posterior border. 
The ventral valve is most elevated in the central third, and 
becomes flattened on the anterior slope; it is somewhat de- 
pressed medially along the raised central third of the value. 
The umbo is depressed and inconspicuous. 
The dorsal valve is flatter than the ventral, is more depressed 
around the sides and has a mesian ridge within, extending about 
two-thirds of the length of the shell from the umbo. The umbo 
is quite low. 
A cast of the interior of the ventral valve, probably of this 
species has pits on each side of the umbo for the cardinal mus- 
cles, on each side of these externally is another pair of larger 
muscle scars; the prints of the adductor muscles appear toward 
the centre of the valve. 
A cast of the interior of a dorsal valve, probably of this spe- 
cies, has two plates extending outward along the slope of the 
valve on each side of the umbo; this plate is connected with a 
sigmoid ridge that rises upward on each side of the slope of the 
cast, the whole forming a crescent similar to that in Obolella 
and Trimerella; the pr ints of the lateral muscles are located by 
these ridges. 
The prints of the adductor muscles. on. each side of the me- 
sian ridge in this valve are very far back. 
Sculptur e. This consists of concentric stris, which are crossed 
by very close, fine, broken, radiating striz ; these cannot be seen 
without a very strong lens. 
Size. Length 13 mm.; width 14 mm.; height of the two valves 
6 or 7 mm. 
