120 UNIVALVES. 



OBSERVATIONS ON THE SHELL AND ITS 

 INHABITANT. 



The shells of this genus are easily distinguished 

 from all others by their conical form and dilated 

 aperture ; hut they are subject to many pecu- 

 liarities, and are therefore divided into several 

 natural groups. Some are remarkable for an in- 

 ternal chamber or partition, which however varies 

 greatly in its appearance. In the Patellae, called 

 the Slippers, it is formed by the columellar lip, 

 which is straight and produced into a flat hori- 

 zontal plate extending half over the mouth of the 

 shell ; these now form a genus called Crepidula. 

 In others, this appendage resembles a cup and is 

 situated at the apex within the shell, which is 

 commonly called from its shape the Cup and 

 Saucer Limpet ; f it is now separated from the 

 Patellae and forms the genus Calyptrae. The 

 apex of many of the Patellae is perforated, J and 

 this peculiarity in the shell is connected with a 

 different formation of the animal, whose gills are 

 situated near the hole through which it receives 

 the needful supply of water ; these now belong to 

 the modern genus Fissurella ; others which have 

 a fissure or notch at the margin are called Emar- 



* Plate V. figures 7, 8, and Plate VI. figure 1. 

 f Plate V. figure 7. Plate V. figure 8. 



