242 SHELLS AND SHELL-FISH. PART I. 



animals merely consists of a cup-shaped protection, with- 

 out the least vestige of a spire, a whorl, or any internal 

 support. The peculiarities of the animals have already 

 been stated, and as those of the shells will be subsequently 

 defined, we may at once proceed to a short statement 

 of their relations and analogies. The whole are consti- 

 tuted of the following divisions, which may be termed 

 genera. 1. Fissurella, having an oval perforation on 

 the apex or summit of the shell. 2. Emarginula, where 

 the perforation is a narrow slit. 3. Hipponyoc, with the 

 shell entire, but reposing on a spurious flattened valve. 

 4. Pedicularia, an entirely new genus. And, 5. Patella, 

 having a simple cup-shaped shell, without any aperture 

 or basal support. The three last are sedentary, and 

 constitute the aberrant genera, the two former being the 

 typical. A very rapid survey of these groups is all that 

 we can give to them. 



(224*.) The genus FISSURELLA has the perforation on 

 the top of the sheU very much resembling a keyhole. 

 This aperture is for the purposes of respiration, as the 

 water thus communicates to the branchial cavity, which 

 is placed, something like that of Doris, on the fore part 

 of the back. The eyes, the mouth, and tentacula are 

 very like those of the Haliotida; ; and the margin of the 

 foot is also edged with short filaments. The shell, ac- 

 cording to Cuvier and Lesson, is placed in the middle 

 of the back, but does not cover it ; so that we can scarcely 

 suppose it is fixed to rocks, like the limpets. The mus- 

 cular impressions in some of these shells, joined to their 

 outward differences, intimate the existence of sub-genera, 

 some of which we have ventured to characterise ; we 

 should not, perhaps, have done this, as the species are so 

 few, had not most of those in the adjoining genus been 

 already named. 



(225.) The EMARGINUL./E, from the very slight notice 

 given of them by Cuvier, do not appear to differ more in 

 their animal, than in their shell, from the last. This 

 difference, however, is quite sufficient to constitute them 

 a genus. Tin aperture in the shell (which is more 



