372 Dr. J. E. Gray on some Families of Bivalve Shells . 



trigonal, veiitricose, 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 very 

 small. 



II. Arc/ina, 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. S/iell 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. Pectunculina, 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, Lamk. Shell suborbicular, rounded 



