TRKNTON LIMESTONE. 207 



273. 5. ENDOCERAS* ANNULATUM ( n. sp.). 



Pl. XLIV. Figs. 1 a, b. 



Cylindrical, very gradually diminishing towards the apex, annulaled by broad rounded 

 ridges, wliicii are equal to tlu; depressed spaces between, and distant from each other one 

 fifth the diameter of the tube, slightly arched upon the back ; surface markings unknown ; 

 septa deeply concave, and bending more abruptly backward just before reaching the 



• Since the name Endoceraa was proposed in the early pages of this report, I have learned from M. de Vekneuii. 

 that the Genus Hyolithes had been previously proposed by M. Eichwald for the embryo tubes of the Orthoceras ; 

 but it does not appear to have been adopted by subsequent writers. M. de Verneuil (Pal. Russia and the Ural 

 Mountains, p. 33U) regards this tube as a mould of the interior of the siphon ; but since we find so many specimens 

 having the embryo tube connected with the jiarent sliell, we can hesitate no longer in our decision regarding these 

 bodies. 



I have felt some hesitation in pressing tlie adoption of the proposed name of Endoctras ; though I have no doubt 

 that when we become better aciiuainted with these curious fossil bodies, such a separation will be admitted. There 

 can be no longer any doubt of the development of the embryo sheatlis within the large siphuncle of many species of 

 the Orthoceras ; while, in others, the siphuncle is evidently too small to admit of such development. That these 

 embryo tubes do contain within them other tubes, which are septate, and which finally become developed into the 

 form of the parent shell, 1 believe can be demonstrated, though we do not yet know all the steps of the process, or 

 whether they are expelled from the siphon of the parent, or take the place of the enclosing body, which decays, and 

 leaves the young and vigorous animal alone. The occurrence of several tubes, one within the other, would induce a 

 belief that they were separable from the parent body without its destruction ; while, in other cases, the existence of 

 a single tube enclosing anotlier difierently marked, suggests the idea that the former may not leave tlie siphuncle. 



From these facts, and from the evidence shown in the succeeding illustrations, it is desirable that any proposed 

 name should include both the parent and embryo tube, as the latter has not an independent existence, but depends on 

 the former, which differs from many of the Orthocerata in the enlarged siphon. 



There are, however, difficulties in the way of a clear separation of the species having this chai'acter, from the 

 ordinary Orthoceras, which has a small siphuncle. In the young specimens which I have had an opportunity of 

 examining, the siphuncle is always small, and it is not easy to conceive of its enlargement to such a degree as to 

 admit of the development of the embryo tubes within it. Owing to these facts, and the possibility of confusion which 

 will result in a multiplicity of species, it may be preferable for the present to indicate the Ktidoceras as a subordinate 

 term under Orthoceras ; noting only such species as are proved to have tlie large siphuncle with enclosed embryo 

 tubes. 



So far as my present knowledge of the Orthocer.\t.\ extends, those possessing the characters given above are 

 confined to the Lower Silurian strata. All those of tlie higher strata, so far as examined, have simple small siphuncles, 

 giving no evidence of enclosed tubes. Should this suggestion prove true in regard to this family of fossils, it may be 

 of importance in its application to the identity of the older Silurian rocks in localities where other characteristics are 

 not satisfactory. 



I am aware that a part of the description given as characteristic of Endoceras, is likewise applicable to the 

 Actinoceras as defined by M. Bronn. But the tubes of Endoceras do not extend throughout the entire length of the 

 sipluincle ; they are cylindrico-conical, filling the siphuncle at its outer extremity, and gradually tapering to a point. 

 The space between this tube and the inner wall of the siphon is usually filled with crystalline carbonate of lime; but 

 the tube is always smooth, and never in any manner presents radiating plates or verticillations. The character of ■. 

 Actinoceras, as given by Bronn, is often seen in the Ormoceras, where a slender ill defined tube of crystalline matter 

 extends through the centre of the siphuncle, with verticillating plates of the same material. These, however, never 

 present, in American specimens, any evidence of organization, and are inconstant in their occurrence even in the 

 same species. The Genus Endoceras, therefore, may be known from Actinoceras, by the internal tube being of a 

 cylindrico-conical form, not continuous with the siphuncle, and having a smooth surface without radii or verticillations 

 connecting it with the walls of the siphuncle. 



