177 

 PROCEEDINGS 



OF 



THE GEOLOGICAL SOCIETY. 



POSTPONED PAPER. 



On a New Genus ©/Cephalopoda, Tretoceras* (Orthoceras 

 bisiphonatum, Sowerby) ; and on the occurrence of the Genus 

 Ascoceras, Barrande, in Britain. By J. W. Salter, Esq., 

 F.G.S., of the Geological Survey of Great Britain. 



[Plate XII.] 



[Read June 4, 1856t.] 



This unique fossil is figured in the ' Silurian System,' pi. 21. f. 23. 

 The anomaly presented by it, as a Cephalopod shell having appa- 

 rently two distinct siphuncular tubes perforating the septa, has always 

 appeared very great ; since, however, so many species with large 

 lateral siphmicles are known {Cameroceras and Endoceras), the ana- 

 logy with these has always offered itself for consideration, and pre- 

 vented inquiry as to which was the true siphuncle, and what might 

 be the nature of the supplementary tube. 



I think, however, that the structure of another and very rare group 

 of Orthocerata will explain the peculiarities of this fossil. 



That the remarkable lateral tube is not produced by the intrusion 

 of any smaller Orthoceras into the cavity of a larger one — a circum- 

 stance very common indeed among these fossils, and one which has 

 been often commented upon J, is evident from the fact that the edges 

 of the septa (a, a) where they abut upon the large tube are decur- 

 rent upon it ; so that the perforation is a natural one. Nor is it 

 comparable with the large lateral siphuncle of Cameroceras, seeing 

 that there exists another and a true siphuncle close to it, and the 

 tube itself is not at all annulated as in the shelly siphuncle of species 

 of the group last mentioned. 



There is one form indeed of Orthoceras (fig. 4) doubtfully referable 

 to the genus (and which Mr. S. P. Woodward considers an evolute 

 form of Discites§, M f Coy, or Nautiloceras, D'Orbigny) which offers 

 a triangular section of the tube, the lateral edges being produced into 



* Diploceras, the name given in the notice of this paper,' Quart. Journ. Geol. 

 Soc. vol. xii. p. 381, has been previously used by Conrad. 



f For the other communications read at this Evening's Meeting see Quart. 

 Journ. Geol. Soc. vol. xii. pp. 369, &c. 



% There is a specimen in the cabinet of the Rev. T. T. Lewis of Bridstow, Ross, 

 in which the tube of a large Orthoceras Ludense is literally crammed with smaller 

 ones ; nearly all the septa had been broken either by their intrusion or before their 

 admission. 



§ Manual of the Mollusca, pt. 1. pp, 86 & 450. 

 VOL. XIV. — PART I. N 



