CHONETES. fia; 



Gemus C'hoiveTES (Fischer, 1837). 



The Genus Chonetes, having the general form of Strophodonta or 

 Strophomena and Lept^na, is distinguished externally from those forms 

 by the tubular spines on the outer margin of the ventral area ; and from 

 Productus 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 



StROPHOMENID^ and PRODUCTIDiE. 



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 divi- 

 sions 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 sepa- 

 rated in like manner by a gi'oove. 



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. 



