﻿BEITISH FOSSIL CEPHALOPODA. 45 



Suborder Ammonitoidea. — The siphuncle is constant, being always filiform and 

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

 points forwards and commences in the centre of a globular first chamber. The 

 sutures are complex, never entirely without lobes, the more acute parts pointing 

 from the aperture. 



Since, with the single exception of a possible G-oniatite, the whole of the Silurian 

 Cephalopoda belong to the first suborder, the classification of the Ammonitoidea 

 will not be at present touched upon. Although the genera of Nautiloids actually 

 founded at the present time are enough, and more than enough, to include all the 

 known forms, the arrangement of these genera in groups so as to indicate their 

 natural relations to each other, which has been attempted by many writers, has not 

 yet to my mind been satisfactorily accomplished. The object is not to make a mere 

 analytical table, without reference to the history of the group ; but to show the con- 

 nection between the relations in structure, and the relations in time. The nearest 

 arrangement to nature is that of M. JBarrande. The character taken by him to be 

 of the highest importance is the shape of the aperture, which he considers gives the 

 shape of the head. This, no doubt, is of extreme importance, and yet, as it seems to 

 me, by a too rigid application of this character we are led to an unnatural separa- 

 tion of forms. In one group, in fact, the contraction of the aperture is associated 

 with other differences, and these together form a natural assemblage ; in another 

 group, the aperture, less markedly contracted, has little influence on other points of 

 structure, and is a matter of comparative indifference. On the other hand, M. Bar- 

 rande does not employ the position of the siphuncle as a classificatory character 

 among the Nautiloids at all, thereby differing from most authors ; but this is accounted 

 for by his not recognising the Clymenias as Nautiloids, and his passing over the 

 TrochoKtes in silence, these being the only genera in which the siphuncle is charac- 

 teristically constant. Without, however, further discussing the grouping of the 

 Nautiloids by other authors, I will proceed to state how it seems to me that Nature 

 herself has grouped them. The earliest, and at the same time the mos't important 

 group, is that of the Orthocerata, whose character is that of extreme simplicity. The 

 curvature is zero, the body-chamber is of the same shape as the earlier part, and the 

 aperture is simple. From the shape of the transverse section, or from peculiarities 

 connected with the siphuncle, the genera Gonioceras, Tretoceras, JEndoceras, Actino- 

 ceras, Bathmoceras, Bactrites, and others have been formed, none of them show- 

 ing such a general departure from the original type as should justify a separate 

 grouping. The simple addition of curvature, which may vary in amount from nearly 

 zero to sufficient to almost make a whorl, introduces us to the genus Cyrtoceras, 

 which runs parallel with the Orthocerata, and is found in almost all the strata in 

 which the latter occur. It has few subgeneric varieties, Trigonoceras being the most 

 peculiar. All these form one natural group, which we may characterise as the 



