114 Annals of the Carne(;ie Museum. 



mens it is much eroded, as is also the outer face of the process. The 

 cardinal process is about as wide as long and bifid. The posterior 

 face is divided only once instead of into four parts as in C. coronaius. 

 At the sides of the process are deep sockets, and in the ventral valve 

 strong striated teeth. The same arrangement for articulation can be 

 seen in a specimen one mm. in length, and seems to undergo no 

 modification during development. 



Muscle Scars. — In the ventral valve, the muscle area consists of 

 two very large flabelliform diductor impressions, between the posterior 

 ends of which are two oval scars of the adductors, one on each side of 

 a thin septum. The interior of the brachial valve is described under 

 C. inucrotiatus. 



Spines. — Individuals over i mm. in length have two pairs of spines 

 on the ventral valve. At 3 mm. in width there are four pairs. As 

 in C. coronatiis the spines near the beak are represented on the adult 

 shell by the bases only. The average distances of the spines, 

 from the beak, are as follows: ist pair, .28 mm.; 2d, .55 mm.; 

 3d, .96 mm.; 4th, 1.41 mm.; 5th, 1.73; 6th, 2.1S; 7th, 2.91; 

 8th, 3.69. 



The spines of this species are more erect than in any of the others 

 here described. They project at angles of from 45° to 70° with the 

 hinge. 



The Same Species front Otiur Localities. 



Specimens from Pratts Falls, N. Y., are, on the average, somewhat 

 smaller than those just discussed. The smallest specimen is 2.2 X3 

 mm. and has 19 striae. The largest is 5.5X8 mm. with 30 stride. 

 The spines extend out at angles of from 60° to 70° with the hinge. 

 On one specimen the outer spines are 2.5 mm. long. 



The material from Eighteen Mile Creek contains many individuals, 

 some of which are larger than those from Canandaigua Lake. The 

 smallest specimen is 4 x 5 nim- and the largest 9X12 mm. The num- 

 ber of striae varies from 27 to 60. An average specimen is 7 x 1 1 

 mm. and has 53 strire. 



The specimens from Thedford, Ont., are of about the same size and 

 shape as the silicified specimens, but have more stride. They range in 

 size from 5X7 mm. to 7 x 9 mm. and an average individual has about 

 44 striae. Many of the adults show all stages of growth, beginning 

 with the nepionic. 



