18 MOLLUSCA FROM THE CRAG. 



lamellated costse of the exterior. One peculiarity, observable in this shell, is the very 

 small portion of surface by which it was attached, its own substance and weight 

 seeming sufficient security against displacement by the movement of the water. 



The form of its muscular impression is elongato-ovate across the shell, differing in 

 no respect in that character from the form displayed by the same muscle in specimens 

 undoubtedly belonging to edulis. The upper valve is quite flat, very thick, and only 

 faintly marked with radiations, scarcely visible in the younger state, and on each side 

 of the ligamental area, upon the edge of the shell, are some denticulations like those 

 visible upon the same valve of edulis in the same place. 



HINNITES.* De France, 1821. 



HINNITA. Ferussac. 1821. Gray, 1826. 

 HINNITES. J. Sow. 1827. 

 HINNUS. J. Sow. 1835. 

 HYNNITES. Herrn. 1846. 



Generic Character. Shell inequivalve, sub equilateral, more or less ovate, thick, and 

 strong, covered externally with somewhat irregular, squamose, or radiating costse. 

 Valves eared with a deep and elongated area for the ligament or cartilage, which is 

 wholly internal ; a large ovate impression by the adductor muscle, that by the mantle 

 entire. 



ANIMAL UNKNOWN. 



This genus was first established by M. de France, in the ' Diet, des Sci. Nat.,' 

 torn, xxi, p. 169, upon a fossil species, which appeared to unite the characters of the 

 two genera, Ostrea and Pecten, differing from the former in adhering by its outer 

 surface, only in its older state, and by the opposite valve : while in the young it was 

 probably fixed by a byssus. It has, by some authors, been united with Pecten since one 

 species of that genus (P. pusio], is in the young state fixed by a byssus, but when more 

 grown, becomes attached by the outer surface of its right valve in the same manner. 

 This peculiar habit is here considered as alone insufficient for excluding the present 

 genus, as the extreme solidity of two or three fossil species of typical characters, with 

 a peculiar form in the muscle mark, seem to indicate a difference in the animal 







inhabitant sufficient to remove them from Pecten. 



In the juvenile state the form of the shell is very similar to that of Pecten with 

 its projecting auricles, and an opening or sinus beneath the anterior one in the right 

 valve through which, in all probability, there issued a byssus ; as it advanced in age 

 its habits became altered, when it fixed itself by the outer surface of its valve. This 

 same habit is adopted by P. pusio in the recent state, though not so in the fossil form 

 of what is considered as the same species in the Crag Formations, where they never 



* Etym. "twin, hinnus, vel uvos, vomer, sec, Herrm. 



