I6 o MOLLUSC A FROM THE CRAG. 



TECTURA,* Audouin and M. Edwards, 1830. 

 PATELLA (spec.) Linn. 

 LOTTIA. J. E. Gray, 1833. 

 ACM^EA. Esch. 1833. 

 PATELLOIDA. Quoy and Gaimard. 

 PATELLOIDEA. Cantraine. 

 HELCION. If Orb. 

 PATELLA (sect.) Lovfai, 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, a 

 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 Acmaea bears also a date of the same year, but Tectura, on the authority of that 

 gentleman, was published in 1 830, 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. 



