TRENTON FOSSILS. 



299 



surface apparently smooth, or girt with slight undulations; septa thin, gently arched, distant from one- 

 fourth to one-twelfth the diameter; siphuncle a cylindrical ventral tube; outer chamber very deep. 



Fig. 212 represents a fragment of the shell of the 0. multicameratum. It is found in the Trenton 

 limestone at Highgate Springs, and in New York it is confined to the Birdseye limestone. 



FIG. 212. 



Orthoceras multicameratum. 



Endoceras (Hall.) This is a provisional name to include those species of Orthoccrata which have a large 

 siphuncle, mostly lateral or eccentric, marked or ridged on the outer surface by the septa, which, from their 

 oblique direction, give it the appearance of a tube with spiral lines. Within this siphuncle are one or 

 more very elongated conical tubes, often one within another to the number of four or five. 

 Endoceras proteiforme (Hall). General form cylindro-conical, more or less elongated, often compressed, 

 tapering somewhat unequally in different specimens; young specimens terminating in an extremely acute 

 point; surface marked by distinct transverse stria?, which usually appear like narrow, subimbricating bands, 

 with one edge well defined and more elevated than the other, more or less distinctly striated longitudinally; 

 strife varying from extreme tenuity to distinct, elevated, thread-like lines ; section circular; septa distant 

 from one-fifth to one-fourth the diameter; siphuncle eccentric or submarginal. 



There is a great variety in the forms of this species. The old and young of the species are generally so 

 united as to form but one shell within another. 



Plate XII, Fig. 1, represents the finest specimen of the Endoceras proteiforme which we have found in 

 Vermont. The long, tapering, slightly curved part of the specimen is the embryo tube ; and the larger 

 part is the parent shell. The septfe are preserved in both. 



This species is rather abundant in the Trenton limestone in Grand Isle, South Hero, and McNeil's Point 

 in Charlotte. 



TRILOBITES OF THE TRENTON LIMESTONE. 



Trilobites are very abundant in the Trenton limestone, and furnish the best tests for its identification of 

 strata in different localities. Certain species of Trinucleus, Ceraurus, Asaphus, and Ogygia are unknown 

 either above or below this rock. Articulate animals, like trilobites, are better tests for the identification of 

 strata than most mollusks ; because the more highly organized the animal, the less is his vertical range. 



Asaphus canalis (Con.) The Isotelus gigas (Dekay) is now referred to the Asaphus canalis, by the 

 best authorities. 



Its general figure is oval-oblong, with the sides rather straight ; buckler in the form of a spherical 

 triangle, obtuse or more or less rounded at the posterior extremities ; cephalic shield convex in the middle, 

 rapidly descending in front and at the sides, margined by a narrow elevated rim or border; eyes sublunate, 

 prominent, subfedunculated, strongly supported on the inner and concave side, by a projection of the 

 glabella ; facial suture continuing from the center of the front, nearly parallel to the margin, until in a 

 line with the eye, where it turns backward, and leaving the eye upon the maxillary portion, turns outwards 

 and backwards, coming out at the base of the shield distant from the angle; thorax with eight articulations, 

 the middle lobe about once and a half the breadth of the lateral lobes, the longitudinal grooves continued 

 slightly into the buckler, and more distinctly into the caudal shield; segments of the middle lobe flat above; 

 those of the lateral lobes, with a groove on their upper surface, extending nearly half way to their extrem- 

 ities; caudal shield of nearly the same form as the buckler, presenting externally some evidence of a 

 trilobite character, sharpened at its upper lateral angles, and having a single transverse groove on each 



