296 



TRENTON FOSSILS. 



FIG. 204. 



nasuta. 



ulations extending nearly twice as far on the posterior as on the anterior side; surface obscurely marked 

 with concentric lines; muscular impressions visible near the anterior and posterior extremities. 

 This shell has been found in Trenton limestone at South Hero and Grand Isle. 



Tdlinomya nasuta (Hall.) Shell transversely elongated, inequilateral; anterior extremity rounded; 

 posterior extremity greatly extended into a kind of beak which is narrowed and compressed, contracted on 

 the base by a shallow sinus; umbones prominent, rounded; shell thin; surface marked by fine concentric 

 lines. The muscular impressions are strongly marked upon the cast, and the umbones are more obtusely 

 angular; there is no evidence of teeth or crenulations. This shell is readily distinguished by the prolonged 

 posterior extremity, which is obliquely truncated and constricted by a sinus on the base. This character is 



more conspicuous on the cast than in the perfect shell; the anterior por- 

 tion of the shell is more gibbous, and regularly rounded at the extremity. 



Fig. 204 represents the right valve of Tdlinomya nasuta. It is a rare 

 species, and is found in connection with the preceding. 



Modislopsis (Hall.) The shell of this genus is equivalve, inequilateral, 

 elongated, becoming broader posteriorly; umbones near the anterior ex- 

 tremity, which is marked by a single strong muscular impression as in 

 Modiola. A sinus often extends from the anterior side of the umbones, 

 obliquely backwards, leaving the anterior portion separated as a kind of 

 lobe. Surface marked by fine concentric striae; shell thin. 



An undetermined species of Modislopsis was found in Trenton limestone in South Hero. 

 Ambonychia (Hall.) Equivalve, inequilateral, compressed, alate or subalate posteriorly, obtuse and ab- 

 ruptly declining or curving downwards on the anterior margin. General form somewhat obliquely ovate, 

 gibbous or inflated towards the umbones on the center of the shell; cardinal margin very oblique, or ap- 

 proaching a line parallel to the direction of the umbones, which are often incurved at the extremity, and 

 equal, or project beyond the line of the anterior extremity; surface marked by more or less prominent con- 

 centric striae, strong undulations, or fine radiating striae. Muscular impressions large, one in each valve. 



The Ambonychia undata (Con.) is the only species of this genus which we have collected in Vermont. 

 It is obliquely ovate or subrhomboidal, with the base rounded, ventricose; anterior margin obtuse, straight 

 above and curving below; posterior margin compressed, scarcely alate above; cardinal line straight, ob- 

 lique; umbones ventricose, elevated, narrowing above and scarcely incurved, with the 

 extremities bending forward; surface with broad, smooth, concentric undulations, 

 which curve downwards more abruptly on the center of the shell; anterior side scarce- 

 ly concave below the beaks; no definite lunette. 



Fig. 205 represents an imperfect right valve of the Ambonychia undata. It occurs 

 in Vermont upon the Island of South Hero. Future investigations may refer this 

 species to some other genus, as the interior of the shell is yet undetermined. 



FIG. 205. 



Ambonychia undata. 



GASTEEOPOD MOLLUSKS OF THE TRENTON LIMESTONE. 



Madurea Logani (Salter.) This shell, when perfect, is fully three and a half inch- 

 es wide, and is conspicuous for the great flatness of its lower or whorled side, and the 

 fewness of its whorls, for, if we except one or two minute inner ones, there are but two or three distinct 

 whorls, which dimmish so rapidly in breadth that the outer is at least thrice the width of the preceding 

 ones in succession, and greater than that of all the inner whorls taken collectively; in M. magna it is greatly 

 less than these. The whorls are very gently convex between the sutures, which are sharply marked though 

 not deep, and are closely striated by regular sharp-arched lines of growth. The sides of the whorl are steep, 

 pyramidal, the depth exceeding the width of the whorl, and are furrowed by a number of deep grooves, 

 sometimes sixteen or seventeen, a few of which are interlined with smaller ones. Occasionally seven or 

 eight only are present, or a deep one occurs at a short interval; but this may be the result of injury. 



