﻿BRITISH FOSSIL CEPHALOPODA. 43 



once was, and which it remains in those comparatively few Nautili which continued 

 the old group into later Neozoic times. Just so among the Corals : the old Rugosa, 

 though said to have their septa in multiples of four, were, in reality, indefinite in 

 the number, while regularity is the character of the newer forms in this respect. 

 Just so among Bchinoids : the old Perisso-echinidce had more than twenty rows of 

 plates ; but the later forms fixed definitely on this number, and their variations 

 passed to the positions of the mouth and anus. Just so in the Crustacea : the number 

 of somites characterises the lower and earlier orders, till, when the number becomes 

 fixed in the Podophthalmia, the variations depend only on the modifications of 

 each. Of none of the other characters can such a statement as the above be made, 

 and thus we arrive at two primary divisions of the chambered Cephalopods. First, 

 those in which the siphuncle is variable in position or character ; and secondly, 

 those in which the siphuncle undergoes no change from species to Species. 



In point of fact, as is well known, in the latter group, which include the 

 Ammonites and Goniatites, the siphuncle is always filiform and external ; but this is 

 no essential part of the definition : there are Orthocerata and Nautili whose siphuncle 

 is so too ; but they do not show the other features of the Ammonites, and hence 

 in them this position and form is only one of the many varieties. 



In addition to these characters of the siphuncle which may be observed in the 

 adult shell, Munier-Chalmers * has shown, and I can confirm to a great extent his 

 observations, that the commencement of the siphuncle shows considerable differences. 

 In the first group it commences at the base of a conical or hemispherical first 

 chamber ; in the second group it begins irregularly in the centre of a globular first 

 chamber, as it does also in the Belemnites. It is thence argued that the Am- 

 monites are not Tetra- but Di-branchiate ; whether this be so or not, their separation 

 from the Nautiloids becomes still more marked, and we are justified in regarding 

 them as belonging to different suborders, in whatever order we may ultimately place 

 the Ammonites. 



Nothing of so great morphological importance distinguishes the Orthocerata from 

 the Nautili, and they cannot therefore be separated into groups of the same value 

 as the above. Barrande makes a third group, intermediate between the Nautiloids 

 and the Ammonites, to receive the Goniatites and Clymenias, which, he says, cannot 

 be separated. It is to be noted that D'Orbigny, whose classification is adopted by 

 Edwards in his ' Eocene Mollusca,' published by the Palseontographical Society, also 

 separates the Clymenias as a distinct family on account of their internal siphuncle, a 

 method of procedure in the highest degree artificial, since it would lead two allied 

 species of Cyrtoceras, or even varieties of one species as C. quasi-rectum, to be classed 

 in two different families. It is for far other reasons that Barrande has made a third 

 family, and these must be discussed with more care. It must be admitted that of all 



1 'Comptes rendus,' torn, lxxvii. p. 1557. 



G 2 



