88 MOLLUSCA. 



on a curved line, the shell is nearly orbicular, and the mus- 

 cular impressions, which are two in number, form each a 

 callous projection with a sharp margin. The area pilosa> a 

 native of Britain, is referable to this genus. 



In the genus CUCULLJEA, the teeth of the hinge are simi- 

 lar to the arcae, but at each extremity there are three or 

 four transverse parallel ribs. It is represented by the area 

 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 TRIGONIA. 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, T. pec- 

 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 Trigonees 

 formed by Lamark at the suggestion of M. Valenciennes. 



14. OSTREA. Linnaeus, 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 gro wth 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 Linnaean genus ostrea compos- 

 ed, which, in spite of all its imperfections, has still its ad- 

 mirers in this country. (See Descriptive Catalogue, Lin. 



