]].S 



Annals of the Carnfjjie Museum. 



and has from 25 to 28 striae and six pairs of spines. Except in a few 

 specimens showing senile characters, the hinge width is the greatest 

 width of the shell. 



Chonetes robustus Raymond. (Plate 7, Rows i and 2.) 



Am. Jour. Sci.. \'ol. X\"II, p. 289, pi. XVII, Rows I and 2, April, 1904. 



This species is represented by specimens in the neanic and ephebic 

 stages only, so a complete developmental series 

 could not be obtained. The smallest specimen 

 is 2.6 mm. long, and 3.2 mm. and the largest 

 1 1.06 X 14-33 mm. 



Neplouic S/ax^. — The beaks of the valves are 

 not well enough preserved to show the outline of 

 the nepionic shell, so its exact size and contour 

 is not known from these specimens. The dorsal 



w r i t e r' s collection. 



X2 



Fig. 2b. ( honetes 

 robmtus Raymond. 

 Ventral valve of a typ- 



specimen rom le ^^^^ j^^g ^ median fold and two lesser lateral folds. 

 This valve was convex at the beak and concave in 

 front. The ventral valve is convex with a nar- 

 row median sinus. In one specimen the dorsal fold extends for. 60 

 mm., and in another .66 mm., so the nepionic shell was probably of 

 about that length. 



Changes Di/n'/io DcvelopDieut. 



Outline. — The outline of no shell less than 1.93 x 2.92 mm. can 

 be made out, so the list of changes is very incomplete. Until the 

 shell reaches 6 mm. in length the index is very constant at about 

 1.40. Older shells have the length and breadth a little more 

 nearly equal and the index falls to 1.20 in the largest shell in the 

 collection. In most of the specimens the width of the shell at the 

 hinge, is greater than the width below, but in two of the largest indi- 

 •viduals it is a little less. 



Convexity of Valves. — The ventral valves of the younger shells are 

 only slightly convex, but in mature specimens they become very 

 strongly so. The dorsal valve is flat or slightly concave in the younger 

 stages, but is strongly concave toward the front in adults. 



Striie. — One of the nepionic characters, the absence of striic, con- 

 tinues into the early neanic stages. In some individuals this smooth 

 state is retained longer than in others. The largest smooth shell is a 

 dorsal valve 3.16 mm. long and 3.4 mm. wide. On another this 



