242 TWENTIETH REPORT ON THE STATE CABINET. 



IV. THE GENUS CHONETES (Fischer, 1837). 

 (From Vol. lY, Palasontology of New-York.) 



The Genus Chonetes, having the general form of Stroyhodonta or StrO' 

 phomena and LeptcEna, is distinguished externally from those genera by the 

 tubular spines on the cardinal margin of the ventral area ; and from Produc- 

 tus, by the manner in which these spines are disposed. In some species, 

 however, in addition to these, the surface of the shell is studded with small 

 spines as in Productus. Internally it has been shown by Mr. Davidson 

 to possess characters which ally it with Productus ; and it connects more 

 nearly, by form and general features, the families of StroyhomenidcB and 

 ProductidcB. 



In the ventral valve, the area is narrow but well-defined, with a distinct 

 foramen which is partially closed by a pseudo-deltidium, and the remainder 

 of the space is filled by a cardinal process from the opposite valve. In 

 the bottom of the cavity there is a longitudinal median ridge, and on each 

 side and close to it are the occlusor muscular impressions : the divaricator 

 impressions occupy a wider space, and usually are not very well defined 

 in their limits, though having the same general form and features as those 

 of Strophodonta. 



In the dorsal valve, the teeth-sockets are well marked. The cardinal 

 process is entirely unlike that of the preceding genera, being simple at its 

 base, and bifid or grooved at the extremity ; while the lateral divisions 

 usually reach nearly as far as the central portion of the process, and are 

 separated from that by a groove more or less distinct. Sometimes the 

 lateral or accessory lobes are distant from the extremity, and separated 

 in like manner by a groove. 



In one or more of the slender forms of the process, these lateral lobes 

 are situated at some distance from the apex, and appear like minute 

 processes on the sides. At its base, the cardinal process is continued in a 

 slender median ridge. The double occlusor impressions are small, and 

 from between them proceeds an elevated vascular marking which assumes 

 a reniform shape ; this being a distinguishing feature of the genus, which 

 allies it with Productus. 



The interior of the shells are strongly pustulose or papillose : the little 

 granules are arranged in the direction of the striae and along the grooves 

 of the exterior striae, whict often become defined elevations upon the inner 

 surface of the shell. 



So far as we at present know the species in the rocks of this country, 

 the genus begins its existence in the shales of the Clinton group, where a 

 single species' has been found. There are two species known in the Lower 

 Helderberg group, and but a single one in the Oriskany sandstone.* Two 

 species are known in the Schoharie grit, and three others are common in 

 the Corniferous limestone ; while others are added to the number in the 



* The same occurring in the Lower Helderberg group. 



