373 Dr. J. E. Gray on some Families of Bivalve Shells , 



trigonal, ventricose, inequilateral, equivalve, strongly costated; 

 hinder slope keeled ; margin closed, strongly folded ; umbo 

 acute; cardinal area narrow, smooth; the anterior hinge-tooth 

 elongated, divided into many subequal plates; the hinder tooth 

 ovate, arched, small, of a few irregular laminae. Periostraca 

 dark, foliaceous. 



I. Noetia triangularis, n. s. 



c. Shell suhcordate, inequivalve; anterior cardinal tooth tery 

 small. 



II. Argina, Gray, Syn. B. M. 1840, 155; 1844, 81 & 91. 

 Shell subglobose, subcordate, subequivalve, inequilateral, strongly 

 radiately costate; umbo subanterior ; lunule none; left valve 

 rather the largest, with the hinder part of the lower margin 

 dilated and more deeply folded ; hinge-area narrow, smooth ; 

 hinder cardinal tooth elongated, curved, the central crest small. 

 Periostraca brown, foliaceous, with flat conical laminse in the 

 radiating grooves. 



a. Shell subglobose. A. pexata. 



b. Shell oblong, elongated. A. Campechensis = A. Americana 



= transversa = compacta, A. pectinoides. King = labiata, 

 A, brevifrons. 



12. Lunarca, Gray, Syn. B. M. 1844, 81. Shell globose, 

 subcordiform, radiately costated, nearly equivalve; umbo sub- 

 anterior ; lunule distinct, compresso-cordate ; the lower hinder 

 margin of the left valve rather the most deeply dentated ; hinder 

 cardinal tooth elongate, narrow in the middle, rather wider at 

 each end ; the front tooth of the left valve ovate, elevated, en- 

 tire, fitting into a cavity in the inner edge of the front margin 

 of the right valve. 



It is possible that this may be only a monstrosity of Argina 

 pexata, as I have observed that the teeth, especially the small 

 anterior one, of this species are very apt to vary ; but I have 

 not seen any bearing a strong resemblance to the specimen on 

 which this genus was founded ; therefore I have retained it until 

 we receive more specimens to elucidate the subject. 



L. costata. 



Tribe 2. Pectuncultna, Gray, Ann. & Mag. N. H. xiii. 417. 



I. The hinge-facet with a broad triangular impression occupied by 

 the ligament, and with the cartilage in angular concentric 

 grooves. 



13. Axinea='Pectunculus,\aimk, Shell suborbicular, rounded 



