404 MOLLUSC A. 



NucuLA, Lam. 



The Nuculae are Arcse, in which the teeth are arranged on a 

 broken line. Their form is elongated, and narrowed near the pos- 

 terior extremity. Their animal is unknown, but is probably not far 

 removed from those of the preceding shells(l). 



This has long been the place assigned to the 



Trigonia, Bnig. 



So remarkable for the hinge, which is furnished with two plates en 

 chevron, crenulated on both faces, each of which penetrates into two 

 cavities, or rather between four plates of the opposite side, similarly 

 crenulated on their internal surface. 



The internal impressions on the shell had already warranted the 

 supposition that the animal was not provided with long tubes. 

 Messrs Quoy and Gaymard have lately discovered living specimens 

 of this genus, and in fact, its mantle, as in the Arcse, is open and 

 without any separate orifice, even for the anus. The foot is large, 

 its anterior portion trenchant and like a hook. 



The living Trigonise resemble the Cardioe in the form of their 

 shell, and the ribs which furrow it: its interior is composed of 

 nacre(2). 



The fossil Trigoniae are different. Their shell is flattened on one 

 side, oblique, longest in a direction perpendicular to the hinge, and 

 traversed in a contrary direction by series of tubercles(3). 



FAMILY 11. 

 MYTILACEA. 



In the second family of the testaceous Acephala, the mantle 

 is open before, but has a distinct aperture for the faeces. 



All these bivalves have a foot, used in crawling, or at least 

 serving to draw out, direct and place the byssus. They are 

 commonly known under the generic name of Muscles. 



(1) .irca pelliicida, Chemn., VII, liv, 541; — Area rostrata, L. ., Id., Iv, 550, 551; — 

 Arc. pella, lb., 546; — ire. nucleus. Id., Iviii, 574. 



(2) The Trigonie nacrf'e. Lam., Ann. du Mus. IV, Ixvli, 1. 



(3) Trig, scahra, Encyc. Method., pi. 237, f. 1; Tr. nodulosa, lb., 2\—Tr. 

 navis, lb , 3;— Tr. aspera, lb. 4- See also Parkins., Org. Rem., Ill, pi. xii. 



