108 AxNALs oi' THE Carnecjie Museum. 



as in O. bellidus. In the adult, dwarfed, 4X5-5 mm. there are 40 

 plications. 



Comparing the development of O. chemu/it^cnsis with that of O. 

 suhplanits of the Niagara we find many differences. 



1. In convexity of valves. The nepionic shells of O. cheftiungensis 

 hive the valves subei^ually convex, which is the adult state in O. stib- 

 plaiiHs, while its nepionic shell is concavo-convex. 



2. In O. .s7//'/'A/-'///j' the cardinal area is low compared with the length 

 of the hinge line and the adults are symmetrical. In O. cheiiuingensis 

 only in the early neanic stages are the shells perfectly symmetrical in 

 most cases, though some specimens with low cardinal areas retain their 

 symmetry into late neanic stages. 



3. In surface characters.* The duration of the nei)ionic stage is 

 about the same in both species. In the youngest specimen of O. 

 subplanus, figured by Beecher and Clarke, 2.25 mm. long, there 

 are seventeen stri?e, six of which are secondary. A specimen of O. 

 chemungensis of similar length has from twenty-eight to thirty-five 

 striae. 



4. Hinge structure. The young of both species have a strong 

 convex deltidium which is prolonged into an exsert tube for the 

 passage of the pedicle. In O. subplanus, the deltidium ceases to grow 

 at an early stage, while in O. chemungensis it continues to increase in 

 size throughout the life of the individual. 



The first three of these differences point to an earlier acquisition of 

 the characters through acceleration. The last would tend to show that 

 O. che/nungensis vfdi'i, in a progressive rather than a retrogressive line of 

 development. 



Chonetes coronatus Conrad. (Plate VI, Rows 3 and 4.) 



Hall, Pal. N. V., IV, 1S67, pi. 21, figs. 9-12. 



This is the least common of the three described species of Chonetes 

 so abundant in the silicified material. About five hundred good speci- 

 mens were obtained, nearly all of them detached valves. Ventral 

 valves are the more common and also the better preserved. All grada- 

 tions from specimens 1.2 x 1.3 mm. to those 15.4 by 23 mm. are 

 found, while from a study of the umbos of the small specimens still 

 younger stages can be made out. 



The smallest specimen figured by Hall is 10.8 mm. long and 14 mm. 

 wide. The largest is 22 by 41.5 mm. 



