138 [Senate 



and often divided into two equal parts by an elevated line along 

 the centre ; entire area strongly striated transversely, and more 

 slightly in a longitudinal direction; inner edges strongly crenu- 

 lated. Surface marked by numerous crowded strise, about nine 

 or ten of which are much stronger and more elevated near the 

 beak of the ventral valve, with j&ner ones between and on either 

 side ; strise frequently increasing by interstitial addition and 

 bifurcation, until they become very numerous and much finer at 

 the margin; strise of dorsal valve similar to those of the ventral 

 valve. In well preserved specimens, fine concentric strise mark 

 the entire surface, but the greater number of specimens do not 

 preserve these markings. 



The interior of the ventral valve presents a double foliate 

 vascular impression without dividing ridge between ; ligamental 

 pits of the adductor muscles strongly marked, and situated 

 close under the arch of the hinge, and separated by a smooth 

 space from the vascular impressions : hinge, upon the under side 

 of the projecting area, and in the place of the foramen, fur- 

 nished with a double dental process which articulates with the 

 teeth of the other valve : dorsal valve with the pits of the ad- 

 ductor muscles near the hinge-line, and just below the inner 

 margin of the hinge furnished with a double tooth, somewhat 

 crescent-shaped, with the horns enlarged and indented at the 

 extremities. 



This process of articulation would leave a small circular space between 

 the projecting portions of the teeth of the two valves, but the margins ot 

 the valves in well preserved specimens are absolutely close and continuous. 

 I have seen, in some specimens, evidence of a minute pore at the apex of 

 the ventral valve, but in many well preserved specimens this does not 

 appear. 



The character of the vascular impression, and the mode of articulation 

 of the hinge, and absence of foramen, in this species, are all so different 

 from the typical forms of Siropho?nena, that I proposed in 1850 the name 

 of Strophodonta. 



At the time of writing the description originally, I had had no opportu- 

 nity of knowing fully the character of the interior of the valves as here 

 shown in the typical species. I am now satisfied that a crenulated hinge-line, 

 alone, is not sufficient to distinguish the species of this genus; but when we 

 have, united with that character, the entire area and absence of foramen, 



