88 MOLLUSCA. 
on a curved line, the shell is nearly orbicular, and the mus- 
cular impressions, which are two in number, form each a 
calleus projection with a sharp margin. The arca pilosa, a 
native of Britain, is referable to this genus. 
In the genus CucuLL™4, the teeth of the hinge are simi- 
lar to the arce, but at each extremity there are three or 
four transverse parallel ribs. It is represented by the arca 
cucullata of Chemn. (Conch. vii. p.174. tab. 55. f. 526-528.) 
To the family arcacea, Lamark, at one period, added 
the genus which he terms Triconra. The hinge teeth are 
only two in number, diverging and compressed, but they 
are transversely grooved on each side. ‘The muscular im- 
pressions are two in number in the recent species, 7. pee- 
tinata, but in some of the fossil shells referred to in this ge- 
nus, Mr. Sowerby could observe only one. The trigonia 
now forms along with castalia, the small family of Trigonées 
formed by Lamark at the suggestion of M. Valenciennes. 
14. OstreEA.—Linneeus, in the construction of this ge- 
nus, brought together many shells totally dissimilar in form, 
character, and habit, and hence it has undergone great al- 
terations in the hands of succeeding conchologists. To as- 
sociate in one genus shells which remain immoveably fixed 
to the rocks and stones from their birth, and which exhibit 
few other signs of vitality than the opening and shutting of 
their valves, with those which possess a locomotive power ; 
to unite such as are irregular in their form and imbricated 
in their structure, with such as are of regular growth and 
solid texture, might surely be regarded as a violation of all 
the laws of a natural or an artificial system. Yet of such 
incongruous materials is the Linnzean genus ostrea compos- 
ed, which, in spite of all its imperfections, has still its ad- 
mirers in this country. (See Deseriptive Catalogue, Lin. 
