SS 
NO. 3 LOOP DEVELOPMENT OF CRYPTACANTHIA—COOPER 5 
After sulcation appears at 2 mm., the sulcus becomes deeper and 
deeper with age. In old adults the fold becomes sulcate anteriorly. 
This produces emargination at the front. This is usually not strong, 
but a few specimens appear that are fairly deeply emarginate. 
From the earliest stage seen to about 3 mm. the delthyrium acts as 
the foramen. After about 3 mm., small triangular deltidial plates ap- 
pear in the lower angles of the delthyrium. These gradually grow to 
restrict that opening. After about 3.5 mm., the deltidial plates be- 
come conjunct and in late adult stages may develop an incipient re- 
flected lip on the posterior margin. The deltidial plates never seal the 
line of junction to produce a symphytium. 
INTERNAL MORPHOLOGY OF CRYPTACANTHIA PROLIFICA 
Changes of great interest and importance take place in certain struc- 
tures inside C. prolifica, especially in the development of the loop and 
the hinge plate. 
Interior of the pedicle value—The valves are firmly articulated and 
the processes of silicification have added to the difficulties of separat- 
ing the shells. Consequently the teeth have not been seen except in 
the articulated state where they appear to be long, slender, and some- 
what curved. 
Dental plates were seen in one of the smallest stages, 1.5 to 2 mm., 
and it is therefore inferred that they would appear also in earlier 
stages. In the youngest stage seen they are receding but vertical. In 
the large adult they are no longer receding, are still vertical but quite 
stout. They are actually slightly advancing at their anterior ends and 
the floor of the delthyrial chamber is somewhat thickened. No de- 
tails of the musculature could be determined. The valves are gen- 
erally thin-shelled and the inner surfaces are not well preserved. 
Interior of the brachial valve—The most important part of the 
brachial valve is the cardinalia consisting of the hinge plates and the 
loop. Like the brachial valve no trace of the muscle scars could be 
determined. The median septum, or ridge, is not a conspicuous feature 
of this genus. 
DEVELOPMENT OF THE HINGE PLATE: In the earliest stage, 1.6 to 
2.0mm. (U.S.N.M. No. 127202k), in which the cardinalia are clearly 
visible (pl. 2, A, fig. 1), the socket plate is a subtriangular ridge fused 
with the crural base on the inner wall of the valve. Buttressing the 
crural base is a small plate, barely visible and converging medially to 
join the floor. These supporting plates do not unite on the floor of 
the valve. Little change could be noticed in any of these plates in 
the next succeeding stages to the 3.1 to 3.5 mm. stage. 
