﻿U BRITISH FOSSIL CEPHALOPODA. 



the Ammonitidse, the Goniatites, especially the group of the Nautilini, are most 

 nearly allied to the Nautiloids ; while the Clymenias, and particularly those which 

 have angular sutures, are nearest among Nautiloids to the Ammonites. Indeed, in 

 the matter of sutures these two genera overlap, and they must necessarily, therefore, 

 be closely allied. They are, however, sharply distinguished by the position of their 

 siphuncle : in the one it is as constantly internal as in the other it is external. 

 Barrande cites the cases of Cyrtoceras and Phragmoceras, to show that this is of little 

 value. These genera, however, have species whose siphuncles are neither internal nor 

 external, and the elements of that organ are either simple or dilated — they have, in 

 fact, variable siphuncles. Whereas the difference between a Clymenia and a Goniatite 

 is not the actual position of the siphuncle, to rely on which would be to repeat the 

 error of D'Orbigny, but that in the latter it has finally settled down into that posi- 

 tion, from which it never afterwards varied. The external siphuncle was a success 

 when associated with complex sutures, whereas the internal one was a failure both 

 in Clymenia and Aturia, both of which were extremely short-lived forms, in spite of 

 these folds in their septa. But are not the Goniatites merely examples of Nautiloids, 

 which, like some others, happen to have this variety of the siphuncular characters ? 

 They might be so considered if they were few in number and of various ages ; but 

 they are the ruling forms in Carboniferous times : their sutures become in some cases 

 almost as complicated as one of the simpler Ammonites, and they possess the same 

 globular first chamber as the latter. It is obvious, therefore, that they are no mere 

 offshoots, as the Clymenias might be, but part of the great evolutionary stem. As 

 the lowest of the Ammonitidse, they have not yet acquired all the peculiarities of 

 the family, and in particular they have a backward and not a forward siphuncular 

 neck to their septa. This detail seems to be in part indifferent, as is seen from the 

 genera Nothoceras and Batltmoceras, which, with all other characters of Nautiloids, 

 agree in this respect with Ammonites (see p. 53). Although Barrande gives the 

 characters of the aperture of the Goniatites as agreeing with the Nautiloids, we 

 meet with all kinds of aperture, except the contracted, both among Goniatites and 

 Ammonites, so that this is not really a distinctive character. The non-persistence 

 of the siphuncular envelope, said to be characteristic of both genera, seems to me of 

 very minor importance. This appears to be the only character which separates 

 Clymenia from the Nautiloids, except the lobes in the sutures, which may be very 

 well matched in Nautilus sinuatus and others. There; is therefore no ground for 

 the establishment of a third group. The two suborders are characterised by the 

 following common features. 



Suborder Nautiloidea. — The siphuncle is variable both in position and structure ; 

 it unites with the septum by a neck which ordinarily, but not universally, points 

 backwards, and commences at the base of a conical first chamber. The sutures 

 are moderately simple, the more acute parts pointing towards the aperture. 



