CYRTOCERAS A PIC ALE. 35 



dorso-ventral diameter of 42 mm., and a lateral one of 38 mm. The position of 

 the siphnncle is the same in both species. 



Of other species described and figured by de Koninck, G. cornu is a more 

 slender shell and has a stronger curvature than the present species. G. digitus is 

 nearer to G. apicale, but it tapers much more slowly ; a fragment only is figured 

 by de Koninck : the species has been identified by him from Rathkeale, near 

 Limerick. Fragments named by de Koninck G. hircinum, G. impotens, C. Nysti, 

 G. ignotum, G. concinnum, and others, must be passed over, as they are too 

 imperfect to make a comparison with the present species in any way satisfactory. 



Remarks. — It is always instructive to consult the pages of Barrande's great 

 work on the fauna of the Bohemian basin, and to study in the rich illustrations 

 to it the varying forms assumed by such an extensive group as the Cyrtoceratida 1 , 

 especially during the period of its greatest development — the Silurian. 



The most striking differences between the present species, which may be taken 

 as typical of the Carboniferous development of the group under consideration, 

 and the Silurian (Stage E of Barrande) forms described by Barrande, are to be 

 recognised in the relative dimensions of the body-chamber, and in the septatiou. 

 Beginning with the first of these characters, it is found that the body-chamber in 

 the Silurian species is generally small, sometimes excessively so, relatively to the 

 size of the shell ; it is often less than one-fourth, sometimes even less than one-fifth 

 of the whole length of the shell (cf. Barrande, ' Syst. Sil. Boh.,' ii, pi. cviii, Gyrt. 

 sequale, pi. ex, G. miles, pi. cxviii, G. acinaces). The septa are, as a rule, crowded 

 together to the utmost extent (cf. Barr.,pl. exxxii, G. nescium, pi. cci, G. Scharyi), 

 indicating a remarkable rapidity of growth in the animal that secreted them. 

 Turning to the siphnncle, it is observable that the beaded character is, in the 

 main, the prevailing one, though this is modified iu form by the curvature of the 

 shell, the position of the siphuncle therein, and the width apart of the septa. 

 Some of the modifications assumed by the siphuncle are well illustrated in 

 pis. exxxiv to exxxviii of Barrande's work. There are not wanting also types in 

 which the siphuncle is cylindrical, as in the living Nautilus, while there are 

 transitional forms from these to the most inflated kinds (cf. Barr., pi. cix, exx — 

 cylindr. ; pis. exxvi, exxviii— transit. ; pis. exxxv, cxlii — inflated). 



Of the Devonian species of Cyrtoceras it may suffice to say that their 

 affinities lean more in the direction of their Silurian progenitors than in that of 

 their successors in the Carboniferous period. The Devonian rocks have not 

 yielded very numerous examples of this type either in Europe or in America, but 

 the different forms it assumes are fairly well represented. Thus we have the 

 short, thick, and quickly increasing shell, exemplified in the Eifelian species C. 

 depressum, Goldf., the type of the genus ; and in contrast with this the slender, 

 beautifully ornamented forms found in the typical rocks of Devon (cf. G. F. 



