GENUS LEPTODOMUS. 339 



MODIOLOPSIS SUBALATUS. 



Modiolopsis subalatus : Hall, Pal. N. Y., Vol. ii, pp. 84 and 285, pi. 27 and 59. 



Some specimens from Racine, Wisconsin, and Bridgeport, Illinois, are 

 apparently identical with this species of the Niagara group of New- York. 

 The specimens are casts, and more or less crushed and imperfect. 



GENUS LEPTODOMUS [?] (McCoy). 



SUBGENUS AMPHICCELIA ( n. g.). 



The Acephala present great difficulties in the way of satisfactory generic 

 reference ; and it is often scarcely possible to arrive at certainty with regard 

 to their true relations. 



A single species from Wisconsin, which is somewhat numerous in indi- 

 viduals, has the general aspect of the more elevated forms of Leptodomus 

 of McCoy ; but it cannot nevertheless be referred properly to that genus. 



The general form of the shell is subrhomboidal, with elevated beaks. 

 The casts present the appearance of a large triangular cartilage pit 

 beneath the beaks ; and just anterior to this, and separated by a thin pro- 

 cess on each valve is an apparent second pit : or the whole may be a large 

 cartilage pit divided by a thin septum. No teeth have been discovered on 

 the extension of the hinge-line. The muscular impressions are faint and the 

 shell thin. 



AMPHICCELIA LEIDYI (n. s.). 



Shell equi valve, inequilateral, somewhat rhomboidal, gibbous except upon 

 the expanded posterior side ; height and width subequal ; umbones gib- 

 bous ; beaks much elevated and incurved, pointed, falling from one-fourth 

 to one-third the width within the anterior margin, which declines from 

 the hinge-line at a very obtuse angle ; hinge-line equalling somewhat 

 more than half the width of the shell. 



The casts show a large triangular pit beneath the beak, and sometimes 

 there is evidence of a thin dividing septum. There are no visible lateral 

 teeth. The surface of the casts is usually smooth, or showing only a few 

 strong lines of growth. In a single specimen preserving a portion of the 

 shell, the surface is marked by fine close radiating striae. 



The height of the shell from beak to base measures in different speci- 

 mens from two to two and a quarter inches, with a width almost precisely 

 corresponding. The depth of the two valves is about one inch and five- 

 eighths. Some smaller specimens, which maybe of this species, have a length 

 and breadth of half these measurements. 



Owing to pressure and other causes, the species exhibits great variation 



in form and proportions. Among the specimens are two with less elevated 



^5 



