﻿42 BRITISH FOSSIL CEPHALOPODA. 



character, together with the constant sinus in their lines of growth on the side 

 in which such a sinus is found in the Nautilus in relation to the funnel, the size 

 of the body-chamber, and the thickness of the shell, are all proofs of their having 

 been tetrabranchiate. By Prof. M'Coy the Bellerophons have been considered 

 Cephalopods, and placed in relation to the Argonaut. There is a superficial 

 resemblance between the shells, and this led Ferrussac, Latreille, the elder Sowerby, 

 and apparently Owen, to associate them as M'Coy has done. It was the same with 

 D'Orbigny; but he confesses that after having seen the shell, which he named 

 Helicophlegma, which has an extreme resemblance to Bellerophon Urii, and which is 

 inhabited by a Heteropod, he could maintain that opinion no longer. In point of 

 fact, however, the resemblance is only superficial, for the shell of the Argonaut 

 being formed by two arms, the line of junction is an irregular one ; whereas in the 

 Bellerophon, there is often a median band having transverse and unpaired ornaments. 

 It was therefore, in all probability, a mantle secretion. 



In excluding the Bellerophons from the Cephalopoda, and in including the 

 Orthocerata among the Tetrabranchiates, most naturalists are agreed ; but there is 

 much difference of opinion concerning the limits and subdivision of the last-named 

 order. In the great work of Barrande all the classifications proposed up to the 

 date of that publication are exposed at full length and passed in review, and it 

 would not be useful to repeat that exposition. The characters on which these 

 classifications have been founded are: — 1. The position of the siphuncle; 2. The 

 form of the sutures ; 3. The involution of the shell ; 4. The form of the aperture ; 

 5. The symmetry or asymmetry of the shell ; 6. The direction of the neck of the 

 septa; 7. The simplicity or complication of the siphuncle. According as one or 

 other of these has impressed an author, it has been made the foundation of his 

 classification, the others being put into the background. By the variation of the 

 order in which they are taken, 5040 different methods of arrangement might be 

 produced, any one of which might be equally right, if the objects to be classified 

 gave us no indication by the history of their appearance and association of what 

 was the true method. But they do give us very remarkable indications of their 

 true relations to each other, and these we must take into consideration to arrive 

 at a natural classification. 



In the earlier Palaeozoic forms we find the siphuncle playing a very important 

 part, and showing many variations. Now it is simple, now it is complicated ; in 

 one it is dorsal, in another ventral, in a third medial. The various species and genera 

 are founded to some extent on these changes ; but when we reach the Groniatites, 

 which at their earliest appeared later than the great bulk of Orthocerata, and from 

 them pass to the Ammonites, the siphuncle shows no variations, it is uniformly 

 small and on the convex side of the shell, and thus ceases to be that important organ, 

 at least for classificatory purposes, and therefore probably for all purposes, that it 





