106 



THE GEOLOGIST. 



process its upper surface is often striated, and afforded attach- 

 ment to the cardinal or divaricator muscles (r). Under this a narrow 



Fig 1. Interior of Dorsal "Valve. Fig. 2. Interior of Ventral Valve. 



a. Posterior occlusor. a'. Anterior occlusor. a. ©cclusor. 



w. Oral prominences (Woodward). r. Divaricator. 



X. Reniform impressions (King). I. Hollows occupied by the spiral arms. 

 j. Cardinal process. 



longitudinal ridge generally extends to about half (or more) of the 

 length of the valve, and on either side are seen the ramified or den- 

 dritic impressions which we consider to be attributable to the adductor, 

 or posterior {a) and anterior (a) occlusor muscles. 



In Producta and in Aulosteges the posterior and anterior divisions of 

 the occlusor muscles (a a) are at times situated so close to each other, 

 on either side of the mesial ridge, as to render the quadruple attach- 

 ment not so distinct as could be desired ; but they are well defined in 

 P. horrida, P. longispina, &c. 



Outside and in front of the muscular scars above described, are the 

 two " reniform impressions " Before referred to [x). Their surface is 

 generally smooth ; they are boimded by ridges, which, after dividing 

 the occlusor muscles, proceed in an outward oblique or almost 

 horizontal direction ; then, turning abrubtly backwards, they terminate 

 at a short distance from their origin. There exists also in many 

 species, but not in all, two prominences {^v), one on each side of the 

 median ridge, and close to the base of the muscular scars. These are 



* In the plates accompanying this article the same letters are used for the 

 corresponding parts as are inserted in the woodcuts. 



