BIVALVIA. 105 



the posterior in the left being the smallest, these in imperfect specimens of the fossil 

 might have been overlooked. 



• The outline of our shell is roundly trigonal, the posterior side being rather more 

 angular and larger than the other, and the diameter is generally greater in a longi- 

 tudinal direction, but in others it is even higher than long ; the right valve has one 

 central, triangular, sub-bifid, cardinal tooth immediately beneath the umbo, and 

 another on each side of it diverging at a very considerable angle ; in the left valve, the 

 three cardinal teeth correspond in form with the interstices of the right valve, two large 

 elongated and elevated lateral teeth, occupy the whole of the dorsal portion of the shell 

 in the left valve, and these fit into depressions of the right one so as firmly to fix 

 the two pieces when they are closed, and on each of these lateral teeth, as also on 

 each side of the dental furrow in the right valve, are numerous fine strife perpendicular 

 to these lateral ridges, and on the inside of the callus or fulcrum for the support of 

 the ligament are the same markings ; the muscular impressions arc somewhat unequal 

 in size, the posterior one being the larger and of a subquadrate form, while the anterior 

 is more triangular, these are connected by the line of the mantle-mark which has an 

 incipient sinus or indentation close to the posterior adductor ; these marks are seldom 

 deeply impressed and not always visible, but when seen, they do not extend beyond 

 the extreme verge of the lateral teeth. On the ouside, the shell is ornamented with 

 numerous, sharp, generally equidistant ridges, parallel to the margin, and in the 

 concave spaces between them may be seen the lines of growth ; a faint line is visible 

 in perfect specimens curving from the umbo on each side, forming a sort of large 

 corselet and lunule, of an elongated ovate form, beyond which the ridges do not 

 extend, precisely similar to what is seen in the Egyptian shell : in most of the 

 specimens of the fossil, the outside is more or less decorticated, but there is very little 

 of erosion visible in any of my specimens at the umbones, nor is the ligament ever 

 preserved, but that is not very thick even in the recent state. 



It has been thought necessary to be thus tediously particular in giving all the 

 minutise of characters belonging to this species, in order to remove any doubt respect- 

 ing its identity with the well-known shell now inhabiting a part of the world where 

 climatal conditions are different from what it is supposed were those under which it 

 existed in this country'. 



A few specimens of this species have been obtained by Capt. Alexander and 

 myself from the Coralline Crag at Gedgrave, near the mouth of the Butley River, but 

 in association with some Helices and other land shells, all identical with existing 

 animals. At this locality, the Crag appears to have been denuded of its more Coralline 

 portion, and these shells are intermixed near the present surface with the remains 

 of the Marine Molluscs of the lower part of that Deposit. 



Depending, therefore, upon this evidence alone, we can scarcely consider the 

 Geological Age of this species to date its existence so far back as the Period of the 

 Coralline Crag. 



14 



