GASTEROPODA. 167 



the recent species. The form of the 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. 



Fissueella,* Braguicre, 1789. 



Larva. Humph. 1/97. 

 Fissurellus. Mont/. 1810. 

 Lucapina. Gray. 



PUPILL^A. Id. 



Macrochisma. Sivains. 1840. 

 Fissuridea. Id. 



Clypidella. Id. 



Fissurellidia. D'Orb. 



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 branchige ; 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 Fissura, a fissure or cleft. 



