190 FIFTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



is depressed into a shallow undefined sinus which is produced in front, and in 

 old shells becomes a linguiform extension. Dorsal valve oval, narrowed towards 

 the beak, a little more gibbous on the umbo than the opposite valve, depressed 

 below the middle, and becoming in old shells abruptly bent upwards Surface 

 marked by close concentric lines of growth, and fine radiating striae are visible 

 upon the surface in the exfoliated shell, and upon the cast. Shell-structure 

 punctate. 



I refer this and the preceding species, with some hesitation, to the Genus Me- 

 RisTA. The)' correspond in general form, and this species has the mesial septum in 

 the dorsal valve, and the two strong dental lamellae in the ventral valve. I have 

 seen no perfect beaks. 



This species is readily distinguished by its greater proportional length and at- 

 tenuation. Some half-grown shells show no evidence of a sinus, while in other 

 individuals it becomes earlier distinct. One specimen measures an inch and three 

 quarters in length, by an inch and a quarter in width : the usual length is from 

 one and a quarter to one and a half inches, with a width of about one inch. 



Geological formation and locality. In loose masses of limestone, south of 

 Young's farm, Williams ville, Erie county. 



MERISTELLA MULTICOSTA ( n.s ). 



Shell ovate, subcircular or transverse, moderately gibbous : beak small, pointed, 

 somewhat incurved. Ventral valve with a broad well-defined mesial sinus, 

 reaching nearly to the beak; in elongated specimens, extended in front. Dorsal 

 valve the most convex; mesial elevation most distinct in the upper part of the 

 valve. Surface marked by strong angular plications, generally bifurcating; from 

 six to ten on the mesial elevation, the lateral ones of which have their outer faces 

 broad, forming the entire height of the elevation; the plications on each lateral 

 portion of the shell about six or eight, variable in number. Numerous concentric 

 wrinkles cross the striae, giving a broken aspect to the surface. Substance of the 

 shell extremely thin. 

 This species differs from M. quadricostata of the upper black shales, in being 



much larger and more robust. The mesial lobe is always distinctly marked; the 



plications are strong, angular, and cover the whole shell. ' 



Geological formation and locality. Shales of the Hamilton group, in numerous 



localities in Western New-York. 



The following species, described under the Genera Obthis and Atrypa, belong 

 to the Genus Meristella. 



MERISTELLA LIMITARIS. 



Orthis limitaris : Vanuxem, Rep. Sd Geol. Dist. New-York, 1843, p. 146, f. 3. 

 Atrypa limit arts ; Hall, Rep. 4th Geol. Dist. New- York, p. 182, f. 11. 



Shell moderately gibbous, subcircular or transverse. Dorsal valve with a broad 



mesial elevation. Ventral valve with sinus only on the anterior portion. Surface 



covered by numerous fine plications, mostly simple. 



When found in limestone, it is full and well formed; but in the thinly laminated 

 shales it is usually compressed, and occurs in great numbers. 



Geological formation and locality. In the black shales at the base of the Ha- 

 milton group, at Leroy and Avon, N.York. 



