106 
THE GEOLOGIST. 
process ;"(y)* its upper surface is often striated, and afforded attach- 
ment to the cardinal or divaricator muscles (r). Under this a narrow 
Lign. 4. — Prcducid horrida. 
Fig 1. Interior of Dorsal Valve. Fig. 2. Interior of Ventral Valve. 
a. Posterior occlusor. a'. Anterior occlusor. a. Occlusor. 
w. Oral prominences (Woodward). * r. Divaricator. 
X. Reniform impressions (King). /. Hollows occupied by tlie 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 Prodiicta 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 bounded 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 (w), 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 tlie same letters are used for tlie 
corresponding parts as are inserted in the woodcuts. 
