160 MOLLUSCA FROM THE CRAG. 



Tectuea,* Audouin and M. Edwards, 1830. 

 Patella (spec.) Linn. 

 Lottia. J. E. Gray. 1833. 

 Acm^a. Esch. 1833. 

 Patelloida. Quoy and Gaimard. 

 Patelloidea. Cantraine. 

 Helcion. D'Orb. 

 Patella (sect.) Loven, 1846. 



Gen. Char. Shell symmetrical, conical, patelliform, generally depressed, and 

 rather thin, often subhyaline and smooth, or faintly radiated upon the exterior ; 

 vertex eccentric, inclining towards the anterior ; base ovate, more or less elongate ; 

 muscular impression, of a horse-shoe form, open in front. 



This genus has been established entirely upon the animal inhabitant, by which 

 alone it can be distinguished ; as in the form of the shell with its radiating marks, 

 as well as in the shape of the muscular impression, it is precisely similar to Patella. 

 The generic position of fossil species can on that account only be determined by 

 their presumed identity with well-known recent shells. The animal, however, is 

 said not to have its branchiae like those of Patella, and it is now placed in a different 

 order. Mr. Gray informed me that his Paper, describing the animal, was read at 

 the Zoological Society in June, 1833, and was published during that year. The 

 name of Acmsea bears also a date of the same year, but Tectura, on the authority 

 of that gentleman, was published in 1830, and of course has the precedence. 



The shells of this genus are generally thinner, more fragile, and more finely 

 striated than those of Patella ; but these characters are variable and insufficient for 

 generic determination. M. d'Orbigny is of opinion that the Patelliform shells of the 

 older rocks were animals belonging to this genus, but the reason for this opinion is 

 not given. 



considered as smooth, with only the visible lines of growth. The diaphragm is largely reflected over the 

 columella, and the outer edge sharp and arched, extending within a short distance of the margin of the 

 shell. This is a dextral shell, and regularly spiral. Specimens of this species, from the Red Crag, are 

 completely enveloped externally, with an agglomerated mass of cellepora ; a probable incrustation when 

 the animal was living, as in no instance has it extended beyond the margin into the interior, which is 

 perfectly clear. 



* Etym. Tectura, a covering. 



