CYRTOCERAS APICALE. 35 



dorso- ventral diameter of 42 mm., and a lateral one ot' -TS mm. The position of 

 the sipbuncle is the same in both species. 



Of other species described and fio'ured by de Koninck, C. cornu is a more 

 slender shell and lias a stronger cnrvature than the present species. C. diijitii.fi is 

 nearer to G. apicale, bnt it taper,'^ niucli more slowly; a fragment onl}- is figured 

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

 Limerick. Fragments named by de Koninck G. hircinum, G. impofens, G. Ni/sti, 

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

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



Bemarhs. — 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 Cyrtoceratidas, 

 especially during the period of its greatest development — tlie Sihn-ian. 



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

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

 and the Silurian (fitage 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, Gijrt. 

 seqnale, pi. ex, G. miles, pi. cxviii, G. ncinaces). The septa are, as a rule, crowded 

 together to the utmost extent (cf. Barr.,pl. cxxxii, G. nesciiun, pi. cci, G. Schari/i), 

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

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

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

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

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

 pis. cxxxiv to cxxxviii 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, cxx — 

 cylindr. ; pis. cxxvi, cxxviii — transit.; pis. cxxxv, cxlii — infiated). 



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 pei'iod. 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 faiidy well represented. Tluis we have the 

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

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

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



