SALTER — TRETOCERAS. 179 



ventral side of the animal is of the same nature as the large lateral 

 siphuncles of the group named Orthoceratites vaginati by Quenstedt, 

 and to one of which Conrad applied the term Cameroceras. 



The posterior part of the mantle in these Orthoceratites must 

 clearly have extended back to the distance of two or three septa, if 

 not to the whole extent of the tube, since the shelly deposit is con- 

 tinuous throughout. And the same must have taken place in the 

 more recent Nautilus (Aturia) Ziczac. This large siphuncle appears 

 from Barrande's account and Hall's figures* to be always filled up 

 posteriorly as the animal moves onward in the shell ; so that the 

 hinder portion of the body, though greatly elongated, does not neces- 

 sarily attain an extravagant length. 



It is, however, quite conceivable, that the hinder portion of the 

 body might be only narrowed and contracted for a given distance, and 

 yet communicate with a siphuncle of the usual form, but which does 

 not elongate, at least in mature age, in proportion to the advance of 

 the animal in the shell. And such, from Barrande's own figures 

 and descriptions, appears to have been the case with the genus Asco- 

 ceras above mentioned ; for, while the terminal air-chambers (and in 

 some species of Ascoceras there were evidently more than one) are 

 furnished with a small siphuncle, the latter does not in any way 

 extend into the incomplete and arched septa which are formed in 

 the inflated portion of the shell, but is continuous with the contracted 

 posterior visceral cavity. In the form we now figure, however, both 

 the enlarged cavity and the siphuncle exist, side by side, and the 

 former is by this means shown to be independent of the latter, and 

 not, as M. Barrande's view of the case would make it, a mere con- 

 tinuation of it. In Tretoceras there must have been a posterior elon- 

 gation or lobe, something like those so commonly met with, of far 

 less size, in the genera Clymenia (where it is dorsal) or in Gonia- 

 tites, and especially in Bactrites (Sandberger), fig. 6, where it is 

 ventral. The concurrence of the siphuncle with the position of this 

 lobe may be only accidental, as in the case of the ventral lobe of 

 Goniatites and Ammonites, while in Clymenia and Aturia Ziczac the 

 lobes are independent of the siphuncle. But in Ascoceras, and espe- 

 cially in those Orthocerata with large ventral siphuncles, the coin- 

 cidence is exact, in the one genus a small siphuncle being attached 

 to the extremity of this lobe, while in the other the siphuncle is con- 

 tinuous with the lobe, and of equal diameter to it. 



In any case the subject of our present notice is worthy of generic 

 distinction, and the name Tretoceras (rp^ros, pierced) seems applicable 

 to its apparent structure. 



Tretoceras, gen. nov. PI. XII. figs. 1-3. 



Elongate, with a subcentral beaded siphuncle, and with the septa 

 pierced ventrally by a wide cylindrical sinus or tube distinct from the 

 siphuncle. 



Species unica. T. bisiphonatum, Sowerby, sp. (Orthoceras) in Mur- 

 chison's 'Sil. Syst.' p. 642, pi. 21. f. 23. 



* Palseont. New York, vol. i. pi. 44, &c. 



N2 



