rKOlMDOLEPTUS FaUNA AT CaNANI )AIf;UA I.AKK, N. \ . 1>5 



tremities have been left free so that the first two pairs form spine-like 

 processes, raised slightly above the surface of the shell. The shell, u\> 

 to the first distinct growth line, has a length of 2 5 mm. and a width of 



5 mm. The spines are each 8 mm. long. The second stage is 3 x 8.6 

 mm. and each spine is 2.5 mm. long. The shell at the last stage is 



6 X 10 mm. and the spines 2 mm. long. So, if the spiniform exten- 

 tions were preserved in ordinary specimens, the outline would be very 

 different from that usually seen in this species (Fig. 13). 



The index, disregarding the alate cardinal extremities, falls from 

 early neanic to senile stages, the length becoming constantly greater, 

 in relation to the width. The extremes are 1.50 and i.oo mm. but the 

 average, in neanic specimens is from 1.30 to 1.45 mm. and, in adult 

 and senile inciividuals from i.io to 1.25 mm. 



Convexity of Valves. — In all stages, up to the later adult and 

 gerontic, the ventral valve is gently and regularly convex, and the 

 dorsal valve flat or slightly concave. Late in the development, the 

 shells become rather abruptly and strongly deflected in front, which 

 makes the ventral valve very convex in this part and the dorsal valve 

 correspondingly concave. The exterior of the shell remains smooth 

 in all stages except for the strong growth varices, of which there are 

 usually at least two or three, and, in gerontic individuals several, 

 toward the front. 



Crenulations on the Hinge. — On a specimen 2x3 mm. the crenu- 

 lations occupy less than half (.45) of the length of the hinge. 

 Those under that size do not seem to have any crenulations. A larger 

 specimen, 5.6 x 7.3 mm., has crenulations which occupy a little more 

 than half (.59). In adults the crenulations cover about this same 

 amount. In one specimen it was .58, and in another .64, which was 

 the highest observed. 



Pedicle Opening. — In the adult, the pedicle opening is very minute 

 and encroaches upon the beak. The deltidium is perfectly flat and the 

 longitudinal striations on the cardinal area extend across without any 

 break. Occasionally the outline of the deltidium is fairly well marked. 

 In young specimens there is a pedicle tube which projects a little 

 beyond the beak. 

 Muscle Scars. 



Pedicle Valve. — There are two large flabelliform diductor scars 

 and, between them, posteriorly, are the two elongate oval ones of the 

 adductors, one on each side of the low median septum. T^ach scar is 

 7 



