FAMILY 10. ARCACEA. 105 



in each valve ; the teeth of one valve closely interlocking with those of 

 the other. The Arcacea are divided into four genera, as follows : 



Cucull^a. 



Arca. 



Pectunculus. 



NUCULA. 



CUCULLjEA, Lamarck. 



Testa subinsequilateralis, trapeziformis, ventricosa, umbonibus distanti- 

 bus, incremento arete ligamenti gradatim discedentibus. Cardo 

 linearis, rectus, dentibus minimis subtransversis, ad extremitates 

 costseforme diffusis. Impressio musculi antici super marginem 

 angulatum, in auriculam productum, imbuta. Ligamentum exter- 

 num, ad partem areae inter umbones affixum. 



Lamarck instituted the genus Cucullrea on account of there being a 

 broad concamerated shelf in the interior of the shell, for the attachment 

 of the anterior muscle. Some authors have refused to acknowledge the 

 distinction, because this character is common, in an inferior degree, to 

 some species of Arca and Pectunculus ; it is, however, especially marked 

 in this genus ; and the area or facet between the umbones, which is 

 formed by the gradual thickening of the dorsal edges, is also very cha- 

 racteristic in its appearance. 



The shell of Cucullsea may be described as rather inequilateral, trape- 

 ziform, and ventricose, with the umbones distant, gradually dividing or 

 receding as the area of the ligament increases. The hinge is linear, 

 straight, composed of a row of small subtransverse teeth, taking at each 

 extremity the form of ribs. The anterior muscular impression is visible 

 upon a sharp edge, produced in the shape of an ear or concamerated 



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