Trenton limestone. 
207 
273. 5. ENDOCERAS* ANNULATUM (n. sp.). 
Pl. XLIV. Figs. 1 a, b. 
Cylindrical, very gradually diminishing towards the apex, annulated by broad rounded 
ridges, which are equal to the 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 Endoceras was proposed in the early pages of this report, I have learned from M. de Verneuii, 
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. 350) 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 parent shell, we can hesitate no longer in our decision regarding these 
bodies. 
I have felt some hesitation in pressing the adoption of the proposed name of Endoceras ; though I have no doubt 
that when we become better acquainted with these curious fossil bodies, such a separation will be admitted. There 
can be no longer any doubt of the development of the embryo sheaths 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, I 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 another differently marked, suggests the idea that the former may not leave the 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 character, 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 Endoceras as a subordinate 
term under Orthoceras ; noting only such species as are proved to have the large siphuncle with enclosed embryo 
tubes. 
So far as my present knowledge of the Orthocerata extends, those possessing the characters given above are 
confined to the Lower Silurian strata. All those of the 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. Bhonn. But the tubes of Endoceras do not extend throughout the entire length of the 
siphuncle; 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. 
