178 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



wing-like crests. It is the Orthoceras paradoxicum of J. Sowerby, 

 from the Carboniferous Limestone of Ireland. Were the back of this 

 species hollowed out a little more, and the wings extended upward 

 in the direction of the dotted lines in our fig. 4, it would present 

 a very close analogy with the section of O. bisiphonatum represented 

 in fig. 3. 



There is also in the Lower Silurian rocks of North America, a 

 genus of Orthoceratidce, Gonioceras of Hall*, which shows the same 

 characters in an extravagant degree (fig. 5). Gonioceras is the most 

 extreme modification in section yet observed among the chambered 

 Cephalopoda. It has a beaded siphuncle, like that of O. bisi- 

 phonatum. If it were less extended laterally, and were the sharp 

 edges curved round in the manner supposed in fig. 4, so as nearly to 

 meet, we should have a form to all appearance exactly similar to that 

 of the internal cast of Tretoceras. I for some time thought that we 

 had in the genus now described (Tretoceras) an extreme form of this 

 kind, and that it would rank next to Gonioceras. But there is one 

 serious objection to that view, inasmuch as there is no indication in 

 the uppermost or terminal chamber of a margin to the supposed 

 excavation. On the contrary, the tube is evidently continuous with 

 that terminal chamber (fig. 1 b), and the argillaceous matrix has 

 flowed from one into the other, without any line of separation, such 

 as must have been the case had the walls of the tube been formed by 

 the outer surface of the shell. 



"We have then clear evidence that this anomalous shell possessed 

 ordinary septa, pierced by an excentric beaded siphuncle, and also 

 had a deep lateral cavity continuous with the terminal chamber and 

 passing down side by side with the siphuncle, — the cavity affecting 

 at least seven of the uppermost septa, if not the whole of them. 



In a late communication to the Geological Society of France, the 

 indefatigable M. Barrande has figured one among the many new 

 Cephalopods of Bohemia, which will, I think, furnish the requisite 

 analogy. He describes Jscocerasf as a flask-shaped cephalopod of 

 the tetrabranch order, with the terminal chamber not only occupy- 

 ing the space above the air-chambers, but extending down one side 

 of nearly the whole length of the shell, in the form of a wide and 

 deep cavity ; this broad cavity being embraced by the decurrent 

 edges of the incomplete upper septa (4 or 5 in number) . It also 

 communicates at its base with a small siphuncle which pierces the 

 minute terminal air-chambers. 



There are no traces of a siphuncle in the incomplete upper septa 

 which are thus formed on one side only of the produced hinder por- 

 tion of the body of the animal %. 



M. Barrande ingeniously argues that this wide cavity on the 



* Palaeont. N. York, vol. i. pi. 14. 



t Bull. Soc. Geol. France, 2nd series, vol. xii. p. 157, and our figure, PI. XII. 

 fig, 7. See also Quart. Journ. Geol. Soc. vol. x. part 2, Miscell. p. 26. 



X Barrande supposes this portion to have contained some of the viscera, so that 

 there would be, on this view, no abrupt distinction between the mass of the animal 

 and the siphuncle itself. 



