PECTUNCULUS. 



l^am, — Hist, Nat. des Anim, sans vert. i, 6. p. 47. 



TESTA orbicularis, sublenticularis, aequivalvis, 

 subaequilatera, clausa. Cardo arcuatus, dentibus 

 numerosis, obliquis, serialibus, alternatim insertis ; 

 medianis obsoletis, subnullis. Ligamentum ex- 

 ternum. 



Placed by Linne and later Conchologists among the Arcs, 

 to which, indeed, the Pectunculi are related in respect of 

 their numerous teeth, arranged in two rows, one on each 

 side of the umbo, as well as in their external ligament; but 

 as these shells have in their general form, and even in their 

 hinge, very sufficient distinguishing characters, it seemed 

 convenient to Lamarck to form them into a particular 

 Genus, which appears very natural, since it detaches a very 

 distinct group, of which there are many species. The 

 Pectunculi are easily distinguished from the Arcce^ not 

 only by the orbicular and lenticular form of their shell, but 

 principally by their hinge, the teeth of which are placed in 

 a curved or arched line, and not straight as in Area; 

 neither are their teeth so numerous or so close set, but 

 larger. The two valves do not gape, and the animal does 

 not attach itself by a byssus, as some of the Areas do ; we 

 say as some of the Arcae do, for all the shells included 

 by Lamarck in his Area are not gaping shells, nor do they 

 attach themselves by a byssus ; and we think with Cuvier, 

 that it would be proper to separate from the Genus Area, 

 those species with distinct ribs, whose erenulated edges 

 shut close and fit into each other, because there is reason to 

 believe their animal is not attached, and rather resembles 

 that of the Pectunculi. Although the umbones of the 

 Pectunculi are not far distant, they are, nevertheless, always 

 separated by an external facet or area, to which the liga- 



