GASTEROPODA. 167 



the recent species. The form of tlie branchial aperture is nearly triangular, with the 

 base upwards, the point extending half way to the margin, with a thickened ridge or 

 plate within, above the fissure, and a muscular band extends round the posterior half 

 of the shell, about one third of its height from the margin ; this muscular band 

 terminates in an extended angular-shaped impression on each side. The form of the 

 base is somewhat of an oblongo-ovate form, the posterior portion being rather the 

 broader. 



Dr. Gould mentions this species as frequently taken in the stomachs of fishes upon 

 the coast of Massachusetts ; and has given, as a synonyme, the Patella apertura, Mont. ; 

 there is, I believe, no doubt of that shell being the young of Fissurella cancellata. 



FissuRELLA,* Bnigidere, 1789. 

 Larva. Humph. 1/9". 

 FissuKELLUs. Montf. 1810. 

 LucAPiNA. Gray. 



PUPILL.EA. Id. 



Maceochisma. Swains. 1840. 



FiSSDRIDEA. Id. 



Clypidella. Id. 



FiSSTJEELLIDIA. VOrh. 



Gen. Char. Shell depressedly conical, with an ovate base ; an aperture in the 

 vertex, on the shorter side, towards the head of the animal ; generally ovate, or of a 

 keyhole shape, sometimes contracted in the centre, sometimes tripartite; rayed or 

 costated upon the exterior, and often cancellated by the thickened edges of the different 

 periods of growth ; margin generally thickened ; muscular impression large, nearly 

 surrounding the interior. 



This genus differs from Emarginula in having the perforation at the summit, 

 through which the water is conveyed to the branchiae ; the opening is on the shorter 

 or anterior part of the shell ; but in the very young state this aperture is nearly 

 central, with a small involute vertex, curving towards the posterior ; and as the animal 

 increases in size, it has the power of absorbing or dissolving a portion of the shell, and 

 the aperture is thereby made larger, and thickened round the edge ; the recurved 

 vertex is then lost, and in the full-grown shell no trace of it remains. From this great 

 difference in appearance between the young and old shells, Montague has made a 

 distinct species of the young state of our Crag shell, which he has named and described 

 as Patella apertura. 



* Etym. The diminutive of Fissitra, a fissure or cleft. 



