ANIMALS. 115 



extremity ; is concave on its upper surface ; and has a ridge passing along its median 

 line. The ridge forms the upper portion of a high vertical plate, which supports the 

 spatula-shaped process, and extends from the under side of the crural base to within a 

 short distance of the centre of the shell (PI. VIII, figs. 3, 4, 5/), and which is evidently 

 the counterpart of the dorsal medio-longitudinal plate already noticed. (Vide 

 ante, p. 70.) 



In order to ascertain the use of certain processes in Camarophoria, it will be 

 necessary to turn our attention for a short while to some other shells of the same 

 class. 



By examining casts of most species possessing dental plates separated from each 

 other, it will be seen that the intervening space between these plates, from the scars 

 thereon, has afforded attachment for certain muscles. Extending our researches to 

 those shells in which the dental plates approximate more and more until they become 

 united, as in certain Strophomenida^ it will be quite evident that the attachment of the 

 muscles has been gradually removed from the surface intervening the dental plates, to 

 that of the plates themselves when they became united. The scars visible on the 

 saucer-shaped process of Leptana analago lead to the same conclusion. Now as there 

 are no muscular impressions on the surface of the large valve of Camarojthoria, and it 

 being certain that its arch-shaped process has resulted from the union at their upper 

 margin of the dental plates, it clearly follows that this arch shaped process has served 

 as a muscular fulcrum. 



Turning our attention to casts of Hypothyris acuminata, or any of its immediate allies, 

 there will be seen on the posterior half of the small valve, on each side of its median 

 line, certain elongated impressions, which are obviously due to muscular attachment : 

 this shell, it must be borne in mind, has no spatula-shaped process, — only the homologues 

 of the two crura of the loop characteristic oi Hypothyris {psittacea). Possessed of these 

 facts, our attention is naturally drawn to the small valve of Camaropkoria : but here 

 we search in vain for any scars corresponding either in form or position with those of 

 Hypotliyris acuminata : they could not occur on the parts where they are situated on the 

 shell last noticed, in consequence of the spatula-shaped process overspreading them : this 

 circiunstance, and the fact that they are not visible on any part not overspread By the 

 spatula-shaped process, or unoccupied by other structures, show that the muscles were 

 nowhere attached to the surface of the small valve : hence it may be safely inferred 

 that the spatula-shaped process of Camaropkoria has been a muscular fulcrum — a 

 conclusion powerfully supported by the elevated apophysis in the small valve of 

 Leptana analago (vide PI. XX, fig. 7 c, d) having been for a certainty a correspondingly 

 subservient structure. 



The vascular system of Camaropkoria appears to be nearly similar to that of 



1 Vide Geology of Russia, vol. ii, pi. xi, fig. 2 a, and pi. xii, fig. 3 g. 



