BARRANDE ON THE SILURIAN CRUSTACEA, ETC. / 



Nautilid^. Gomphoceras 16 



Nautilus 5 Phragmoceras 12 



Lituites 4 Ascoceras 6 



Gyroceras 4 



Trochoceras 22 Ammonitid^. 



Orthoceras 130 Goniatites 6 



Cyrtoceras 80 



Total... 285 species. 



Were I, however, to proceed according to the method of some 

 palaeontologists, I could easily double the number of genera. Thus, 

 in Cyrtoceras there are species with a dorsal siphuncle, others with a 

 ventral, and others again with a central siphuncle ; which would be 

 sufficient with some authors for the establishment of three genera. 

 But I believe that science would gain nothing from such minute sub- 

 division ; and since no better generic characters present themselves, 

 I keep these forms together. The same holds good of the genera 

 Gomphoceras and Fhragmoceras, in which the position of the siphuncle 

 is subject to considerable variation. For similar reasons to the above, 

 I keep together, under Orthoceras, all the straight forms, whatever 

 position the siphuncle may take, and no matter whether it be cylin- 

 drical or swollen between the septa. Indeed I should much rather 

 be disposed to lessen the number of the genera of these Cephalopods ; 

 since most of the above-mentioned ten types afford transitions one 

 into another ; that is, there are species combining characters that have 

 been hitherto ascribed to different genera. Thus, among my Ortho- 

 cerata, there are several species, all the specimens of which are 

 slightly curved towards the point, the rest of the shell being quite 

 straight. Other species present when in fragments the aspect of 

 Orthoceras, but in a perfect condition they have the form of a flat- 

 tened arch. Lastly, there are others which are always very short 

 and conical, like the phragmocone of a Belemnite, and are straight on 

 one side and arched on the other. Should these forms be arranged 

 with Cyrtoceras, on account of their curvature ; or, regardless of this, 

 should we leave them with Orthoceras, with which they agree in 

 other respects — in general aspect, the circular section, the central 

 siphuncle, round aperture, &c. 1 This is a qitestion which I can- 

 not answer, and which I leave to better judges. At all events, these 

 species form a transitional group between the genera Orthoceras and 

 Cyrtoceras. Another instance : — among my species of Trochoceras 

 there are some with a high and turreted spire, and there are others 

 having similar general characters, but with so flattened a spire that 

 we should be inclined to connect them with Lituites or Gyroceras, 

 if their habit were not clearly that of the turreted forms. 



I could enumerate many such examples, where one genus presents 

 a transition into another ; but reserving these for my Second Volume, 

 I will confine myself here to the extraordinary resemblance of the 

 forms among, and, so to speak, the perfect similarity between, the 

 Nautilus and Goniatites of my Silurian rocks. In the Nautilus, the 

 siphuncle is subcentral ; in the Goniatite it is dorsal ; but, if we ex- 

 cept this character, there are sjx'cies of both types presenting a similar 

 aspect in the general form, as well as in tlie ornamentation of the 



