FRESHWATER AND LAND SHELLS. 59 



an inch, and presents the only perfect apex of a spine. This apex is 

 rounded, perfectly smooth, and covered over with epidermal matter. 



The fourth pair is nearly three-fourths of an inch from the third, 

 and the spines are much thicker and stronger, and are both rather over 

 half an inch long; that of the left valve bifurcates near the top, and a 

 little lower down there is the rudiment of a third branch ; that of the 

 left valve presents no bifurcation, but has the rudiment of a branch at 

 the same height as that of the other valve. All the spines are flattened, 

 the greater diameter being transverse — their upper sides are wrinkled, 

 and particularly those of the inferior pair — their lower sides flattened 

 and smooth. This is exactly what we might expect in the formation 

 of these spines. They are constructed like the tubercles on the nodu- 

 lous shells and the horns on the cornutiis. The border of the mantle 

 having deposited the calcareous matter, and the epidermal matter over 

 it, outside of the plane of the shell, where the base of the spine is placed, 

 continues to do this until the apex of the spine is reached. We would 

 then have the larger longitudinal part finished, with its epidermis com- 

 plete, and the centre would form a kind of channel to the apex. This 

 being reached, the mantle forms the same operation, descending in the 

 lower side, filling up the channel, and covering the deposit with epider- 

 mis, and presenting somewhat the appearance of a cicatrix. All the 

 spines which are broken display the remains of the channel, and at first 

 view might be thought to be perforate. This, however, is not the case ; 

 the interior of the valves indicating no appearance of the position of 

 the spines, except a little roughness below the last pair. 



The teeth of this shell differ in some of their characters from any 

 Uhio with which I am acquainted. The highest part of the dorsal 

 curve is equidistant from the extreme ends of the two teeth. The car- 

 dinal teeth are remarkable for their form and position, being very much 

 enlarged at the anterior end, and pointing to the anterior margin. The 

 lateral teeth are lamellar, and unusually enlarged on the inferior por- 

 tion — thus forming an inverted curve, the inferior portion of the double 

 lamellar tooth being very much larger than the superior one. 



The nacre of the shell is purple, approaching a rose colour, and is 

 very beautiful. The beaks being slightly eroded, renders it unable 



