TRENTON FOSSILS. 



297 



Flo. 206. 



The base itself is smooth, or with faint concentric striae only, and the umbilicus (so called) rather 

 abrupt and very narrow, not above one-third the width of the whorl, and with a rounded edge. The shell 

 is solid, nearly a line thick. 



The most singular part of the shell is its operculum, sometimes seen fixed in its normal position, and 

 at other times drawn within the shell. It is exceedingly solid, the successive layers are sub-spirally 

 arranged, tiling over one another, and are antiquated in growth. The nucleus is near the inner and lower 

 angle of the mouth ; in old shells it is pushed further out, and becomes the apex of a very solid short cone, 

 one surface of which lies close upon the inner flat surface of the whorl. Two curved furrows radiating 

 from the nucleus, divide the surface into three areas, less distinct in the mature shell. Inside, a thick 

 compressed process takes its rise beneath the nucleus, retaining its place near the inner angle of the mouth, 

 even in the adult shell, when the nucleus itself has removed further out. The process is as broad as long, 

 and on its oblique free margin it is roughened and grooved, for the attachment of nucleus. Another 

 attachment, similar, but much less prominent, exists at the inner and upper angle, and a line of minute 

 prominences partially connects the two. 



This species is closely allied to the M. magna of the Chazy limestone. It occurs in the Trenton lime- 

 stone at Highgate Springs. We believe this fossil has as yet been found only on the Ottawa River in Canada, 

 and in Vermont. 



A shell related to the genus Euomphalus was collected at South Hero from the Trenton limestone, but 

 was not sufficiently distinct to reveal its specific characters. We believe that the genus Euomphalus is not 

 found in the Trenton limestone elsewhere. 



Bellerophon (Monfort.) Shell symmetrically convoluted, globular or discoidal, strong, few-whorled, 

 whorls often sculptured; dorsally keeled; aperture sinuated and deeply notched on the dorsal side. 



Bellerophon bilobatus (Sow.) is involute, subglobose; height and width about equal; aperture bilobate, 

 large, subreniform; surface marked by fine striae, which, ascending from the 

 umbilicus, form a broad arch on the side of the shell, and, bending downwards, 

 meet in an abrupt curve on the dorsal line. 



This species is represented in Fig. 206. It is abundant in the Trenton lime- 

 stone, occurring at almost every locality. We have seen it at Larrabee's Point, 

 at McNeil's Point, in Bridport near the village, in South Hero, at the north end 

 of Isle La Motte, and at Highgate Springs. 

 Cephalopod Mollusks in Trenton Limestone. 



Trocholites, (Con.) Shell discoidal; whorls close or separate; last chamber 

 produced in a straight line; siphuncle central. This genus is probably synonymous with Lituites. 



Trocholites ammonius (Con.) This species was found by Prof. Adams in Vermont, in Addison County. 



It is discoidal; volutions in the same plane, about four, rounded, slightly 

 concave on the ventral side, gradually enlarging in size towards the aper- 

 ture, which is slightly expanded; surface marked by lamellose, irregular 

 and oblique transverse striaa or ridges, between and upon which are finer 

 lamellose strise covering the outer surface, and giving it a peculiar textu- 

 ral or netted appearance; striae meeting in an arch upon the back; septa 

 direct, or slightly undulated on the dorsal side; outer chamber large; 

 siphuncle ventral. 



Fig. 207 represents a large specimen of Trocholites ammonius, which 

 preserves the lamellose surface. 



Orthoceras (Breyn.) Shell straight; siphuncle central; aperture 

 sometimes contracted. The orthocerata are the most abundant and wide- 



Trocholites ammonius. . , 



spread shells of the older rocks, and have attained a larger size than any 



other fossil shell. Specimens have been found twelve feet long and a foot wide. Such carnivorous monsters 

 must have been in the silurian seas what sharks are in alluvial oceans. 

 20 



FIG. 207. 



