74 PALJEONTOLOGY OF NEW YORK. 



Diagnosis. Shells having the same general external characters as Mebista. 

 Valves convex, often inflated, cardinal areas obscure. The umbo of the pedicle- 

 valve is incurved at maturity, concealing most, if not all of the foramen ; in 

 early stages of growth, however, the beak is more erect and exposes the delti- 

 dial plates in an elementary condition of development. The anterior margin 

 of the shell is sinuate, and usually there is a sinus on the pedicle-valve, with a 

 less conspicuous fold on the brachial valve ; sometimes both valves bear a low 

 sinus, or the sinus on the pedicle-valve may be absent, while the fold on the 

 brachial valve is present, thus giving the shell a nasute anterior extension ; 

 again, fold and sinus may be absent on both valves. 



In the interior of the pedicle-valve the delthyrium is wide, its margins being 

 thickened into dental ridges. The teeth are conspicuous, often much thickened 

 and curved backward at their tips, interlocking with the opposite valve in such a 

 manner as to make a very firm articulation. The teeth are supported by lamellas 

 which rest upon the bottom of the valve, and are continued for a short distance 

 about the posterior margin of the muscular impression. In old shells this por- 

 tion of the valve becomes greatly thickened, the muscular impression corres- 

 pondingly deepened, and the identity of the dental lamellae is obscured by 

 their becoming merged with the substance of the valve. The pedicle-cavity 

 is deep and frequently shows a strong muscular scar. The impression of the 

 diductor muscles is subquadrate-ovate or subtri-angular in outline, very strongly 

 impressed and usually clearly divisible into its two lateral components. The 

 central adductor scar is faint, but linear when retained. The lateral scars are 

 deeply striated longitudinally. The anterior margin of the muscular area is 

 frequently obscure but is not infrequently a ridge from which radiate fine, 

 anastomosing pallial sinuses. In the post-lateral regions the ovarian sinuses 

 are sometimes retained. 



In the brachial valve the beak is depressed and sometimes obscured by the 

 incurvature of the umbo of the opposite valve. The dental sockets are nar- 

 row and divergent. The hinge-plate is subject to some unessential variation 

 in form. Usually it is triangular, concave on the upper surface, and divided 

 into two lobes by a median groove. The crura take their origin from just 



