﻿52 BPJTISH FOSSIL CEPHALOPODA. 



membrane forming the covering of the bulb is not only thrown into folds, which 

 reach from septum to septum, but these folds pass into the body of the bulb along 

 the central line, and are seen when the bulb is split in half, as is often the case with 

 Carboniferous examples, to end round a central circle. There could be no drawing 

 in in this way if there were no internal tube to hold to. In some examples also, not 

 as yet recognised among British fossils, there are vertical lamellae passing inwards, 

 and these also must have had some internal support. We know that the living 

 Nautilus has two envelopes to its siphuncle, and in Actinoceras they were both of a 

 character suitable for preservation. It must be admitted that other genera of 

 Cephalopods have similar structures, and that logically they ought to be separated 

 into subgenera ; but we here reach questions of convenience merely. The names 

 Ormoceras and Huronia are founded on mere variations in the form of the siphuncle 

 or in its internal structure, and may well be merged in Actinoceras. 



2. Endoceras. — In addition to their large and usually 1 lateral siphuncle, the 

 Endocerata possess the peculiarity that the neck of each septum is continued 

 backwards into the cavity of the next previous one, so that the siphuncle has a 

 complete shelly envelope. Within this large siphuncle is often found some smaller 

 Orthoceras, with its aperture in the same direction as that of the enclosing one. 

 These are often covered by a smooth deposit fitting over them so as to be conical. 

 This latter is supposed by Hall to be a sheath, which he calls hence an " embryo 

 sheath." In some examples there are more than one of these, each fitting into 

 the previous sheath. The base of the siphuncle is often filled with a calcareous 

 deposit, which is also composed of conical sheaths, but which often shows no signs 

 of structure, being solid throughout. Barrande considers the " embryo sheaths " to 

 be merely casts of the part of the siphuncle not filled with organic deposit — an idea 

 which is rather difficult to reconcile with the existence of several sheaths one within 

 the other — unless these outer ones are made of the organic deposit, and not of the 

 infilling of the siphuncle, in which case only the inner one can be the cast ; on the 

 other hand, the so-called embryos do not appear, by the size and character of their 

 siphuncles, to be of the same species as their so-called parent. To this subgenus the 

 fossils described by Billings as Piloceras belong, being undoubted Orthocerata, with 

 similar siphuncular deposits. 



3. Tretoceras. — One part of the body-chamber, occupying, in the type, a lateral 

 position, is prolonged backwards for the distance of several septa, but for how many 

 is unknown, as is also its termination. The earlier septa are slightly dragged back by 

 this so as to form a lobe in its neighbourhood ; but as the prolongation passes by the 

 septa, it would appear to have been attached to the shell independently of them. 

 This has nothing whatever to do with the siphuncle, which in the type is subcentral 

 and nummuloid, but might obviously have any other character. The siphuncle of a 



1 Barrande describes some with subcentral siphuncles. 



