90 Mr. W. King on certain Genera 



opposite valve, as in Pentamerus galeatus. Two condyle plates 

 pass from the deltidium, one on each side of it, to a third of the 

 length of the shell; they conjoin at their superior margin, so 

 as to form an arch-shaped process, the crest of which is attached 

 to the roof of the valve by means of a shallow vertical plate. 

 In no respect do these plates differ from those composing the 

 arch and its support in Pentamerus ■, except in degree. 



In the ventral valve, the space between the socket-walls is 

 occupied with a triangular horizontal plate or platform, having 

 two of its margins attached to the hinge, and the other one free 

 and facing the cavity of the shell. Upon the platform is situated 

 a rounded protuberance, which from its position and the lines or 

 striae on its surface, is evidently the cardinal muscular support. 

 From the free margin of the platform arise two slender filiform 

 processes (one on each side of and close to its centre), which 

 curving upwards pass to the anterior end of the arch, just within 

 touching it. Immediately below these processes, a much larger 

 one is seen to originate, and to project with a slight upward curve 

 nearly to the centre of the shell, and within a third of its own 

 length of the opposite valve : it becomes considerably dilated 

 towards its free extremity, and is concave superiorly, which 

 gives it a spoon-shaped appearance. This process is supported 

 by a deep vertical plate extending from the under side of the 

 platform to a considerable distance along the medio-longitudinal 

 line of the shell. 



On comparing the armature of the dorsal valve of Camero- 

 phoria with that of the corresponding valve of Pentamerus, the 

 strongest resemblance is visible ; but as the arch and its support 

 are occasionally seen in other genera, they cannot be considered 

 of much value in a generic point of view : if however our atten- 

 tion be directed to the ventral valve, we observe a structure 

 which cannot be disposed of so summarily. In Pentamerus the 

 separation of the two socket-plates at their base is such as to 

 afford room for the attachment of the inferior terminations of 

 the valvular muscles to the inner surface of the lower valve. In 

 Camerophoria however I am convinced that these muscles were 

 not so attached, but that they were supported by the projecting 

 spoon-shaped process. The reasons for this view are, that no 

 muscular impressions are visible on the inner surface of the 

 ventral valve; that where they only can occur, the surface is 

 crowded with vascular impressions ; and that in Hypothyris, an 

 allied genus, several species (H. acuminatus, H. pugnus, &c.) 

 exhibit the muscular impressions on that part of the inner surface 

 of the ventral valve corresponding to the place in Camerophoria 

 which is overspread by the spoon-shaped process. Has a valvular 

 muscular support, in the form of a projecting process, ever been 



