﻿NORWOOD AND PRATTEN ON CHONETES. 29 



slightly concave, leaving but a very small space for the lodgment of the animal. The 

 cardinal edge has six tubes on each side of the beak. Its surface is covered with rather 

 large ribs, which become nearly obliterated on the ears. The ears are small and 

 somewhat flattened. At the distance of five millimetres from the beak there are 12 

 ribs in the space of five millimetres, and the same number in the same space on the 

 margin ; they are fourteen in number when they leave the beak, but increase by bi- 

 furcation, so that when they arrive at the margin there are nearly sixty. 



Ventral valve.— -Its ribs are in all respects like those of the dorsal valve. 



Dimensions. — Length, 12 millimetres; breadth, 15. 



Geological Position and Locality. — This species occurs in the Devonian limestones of 

 the Devil's Backbone, Jackson county, Illinois. Illinois State Collection. 



Explanation of the Figures. — PI. II., fig. 9, a. View of the dorsal valve. 



Fig. 9, b. View of the ventral valve and area. 



Fig. 9, c. Longitudinal section. 



C. Martini, nob. — PI. II., fig. 10, a, b, c. 



Shell of medium size ; depressed ; transverse ; sub-quadrangular ; the cardinal border 

 being, in a slight degree, the broadest portion of the shell. Dorsal valve somewhat 

 convex, and covered with tolerably large ribs, which bifurcate two or three times 

 before attaining the margin, where the whole number amounts to eighty. It has a 

 wide but shallow sinus, which originates near the beak, and gradually widens as it 

 proceeds towards the front. Ears flattened. Area unknown. The cardinal edge has 

 eight thick blunt tubes on each side of the beak. 



The ribs of the ventral valve are like those of the opposite one. On its internal 

 surface it has a cardinal tooth supported on four plates, which project inwardly, and 

 appear to be formed by the union of several ribs. The two plates nearest the cardinal 

 edge are much the largest. The internal ribs, which correspond to the external fur- 

 rows, show a series of granulations, but they are farther apart than in most of the 

 species of the genus. 



Dimensions. — Length, 22 millimetres; breadth, 25. 



Ribs. — There are, on the margin, eleven in the space of five millimetres. 



Comparisons and Differences. — This species differs from the C. Maclurea in its more 

 quadrangular shape, in the smaller number of its ribs, and in the possession of a sinus, 

 (that species having none.) From the Tuomyi it differs in its sinus, in the greater 

 number of its ribs, and in having eight tubes on each side of the beak, while the 

 Tuomyi has but six. 



Geological Position and Locality. — This species occurs in the Devonian limestones of 

 the Devil's Backbone, Illinois. Illinois State Collection. 



Explanation of the Figures. — PI. II., fig. 9, a. View of the dorsal valve. 



