TROPinoi.EPTUS Fai'xa at CANAXDATcrA Lakk, X. ^'. 121 



nepionic shell varies considerably in the different species, but is 

 always less than i mm. It is least in C. coronafiis and greatest in C. 

 mucronatus . 



Pauciplicate Neanic Stage. — In this stage new stria: are intro- 

 duced by implantation and not by the bifurcation of the older strias. 

 The strice are strong, simple, and separated by spaces as wide as the 

 stria;. The number of the striae is not great. In C. coronatus and 

 C. scituliis the largest number is usually 13 on the ventral valve and 

 12 on the dorsal. In the other species there are more, but usually less 

 than 20. 



The shell is about one fifth wider than long, and gently concavo- 

 convex. The first spines (usually two or three pairs) appear during 

 this stage. The size of the shell at the end of this stage, that is, 

 when the stri^ begin to bifurcate, is least in C. scitulus, where it is 

 about 2 mm. in length, and greatest in C. rohushis, where it reaches 

 a length of 6.5 mm. 



Later Neanic and Epliebie Stages. — It would be difficult to make 

 any sharp line which would mark the end of the adolescent period 

 and the assumption of all the adult characters. The size, convexity 

 of valves and number of striae continue to increase during these 

 stages ; the width becomes greater in proportion to the length and a 

 number of pairs of spines are added on the cardinal margin. 



Gerontic Stage. — Senile characters are not well shown except in 

 C. mucronatus. In all, this stage seems to be accompanied by a 

 thickening of the shell, a deepening of the muscle scars and a growth 

 of the anterior part of the shell, which reproduces the early neanic 

 condition of length almost equal to breadth ; and an increase in the 

 convexity of the ventral valve. 



The simplest type of Chonetes would then seem to be one with a mod- 

 erately convex ventral valve, the width a little greater than the length, 

 the pedicle tube open at maturity, the surface covered with simple 

 striae and with few pairs of spines. Of the species here discussed C. 

 7-obustus is the most primitive in its character, as is shown by the con- 

 tinuation of the smooth state into the early neanic stage, the large 

 size of the shell and the comparatively large number of striae acquired 

 before the end of the " pauciplicate " neanic stage, and the retention 

 of the open pedicle tube in the adult. 



Of the species in the older formations, C. cornuta Hall of the Clin- 

 ton and C. undulata Hall of the Niagara show some of the primitive 



