FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 101 



Genus Meristina. Han. 



MeriaKna, Hall. 20th Regent's Keport 1867. 



Etymology: Merista, a genus of shells; inus, implying resemblance. 



The shells of this genus resemble in shape and general aspect those of Mer- 

 istella ; they differ only from the latter by internal arrangement of the loops. 

 The lamellae of the shells of this genus are united by a single loop only. This 

 interior arrangement is observed in Meristina maria and M. nitida, both of the 

 Magara group. In neither of these species is there any indication of acces- 

 sory lamellae, as in Athyris, nor evidence of the extension of the loop beyond 

 a certain point. The simple character of the spires in these forms are of 

 sufficient value to constitute a new genus, that of Meristina. 



Meristina maria. HALL. 



Plate XXIX., figures 7, 8, 9 and 10. 



Mcristella maria, Hall. Trans. Alb. Inst., Vol. IV., p. 2121863. 

 Meristina maria, Hall. Pal. N. Y., Vol. IV., p. 2991867. 

 Meristina maria, Hall. 28th Bep. N. Y. St. Mus., Mus. Edit., p. 1591879. 

 Meristina maria, Hall, llth Geol. Kep. of Ind., p. 2991881. 



Shell of large, at least of more than medium size, ventricose, broadly ovate 

 or subquadrangular ; length and width usually equal ; greatest width at one 

 third of length from base or front. Ventral valve gibbous in the upper half, 

 having a slight elevation from beak to middle of valve, where it becomes de- 

 pressed and sinuate, and abruptly deflected upward into a linguiform basal 

 extension, from the point of greatest convexity, which is a little above middle 

 of valve; it slopes gently to lateral and basal margins, but more abruptly to the 

 cardinal margins, at which the cardinal borders are inflected. Umbo is prom- 

 inent, and beak strongly incurved upon the umbo of other valve. Dorsal valve 

 gibbous, greatest convexity below umbo, from where it slopes abruptly to 

 cardinal margins, but in a gentle curve to the lateral and basal margins ; a sub- 

 angular ridge extends from beak to front, near which it increases rapidly in 

 width and elevation, thus forming a prominent but undefined mesial fold ; 

 umbo moderate, and beak strongly incurved under that of other valve. 



Surface marked by strong, concentric lines of growth. Interior of ventral 

 valve marked by two strong diverging dental lamellae, which extend to near 

 the middle, limiting a deep, triangular muscular cavity. 



This species is most nearly related to Meristella (Merista) tumida of Euro- 

 pean authors, but is less rotund, while that species does not possess the pecu- 

 liar flattening of the cardinal half of the ventral valve, and its accompanying 



