44 CARBONIFEROUS CEPHALOPODA OP IRELAND. 



fairly approximate, about 5 mm. distant from each other in the vicinity of the 

 body-chamber. Siphuncle near the margin of the ventral, flattened side ; strongly 

 inflated between the septa. Test perfectly smooth. 



Size. — Length of the most perfect specimen (wanting anterior portion of 

 body-chamber) 160 mm., length of septate part 120 mm., length of portion of 

 body-chamber preserved 50 mm. 



Affinities. — The plano-convex form of the shell and the more numerous septa 

 are features that readily distinguish this species from the one next to be described, 

 viz. Poterioceras latiseptatum, Foord ; its much more slender proportions separate 

 it unequivocally from Poterioceras cordiforme, J. Sowerby. 



Remarks. — A specimen of this species having been obtained with the apex 

 nearly perfect, a much better conception of the shape of the shell can now be 

 formed than has been possible hitherto. The extreme apical point is unfor- 

 tunately broken in the specimens available, so that nothing can be ascertained 

 with reference to the presence of a cicatrix or other embryological mark. 



Through the kindness of my friend Mr. G. C. Crick, of the Geological 

 Department of the British Museum, I have been enabled to reproduce a drawing 

 he made for me of the posterior end of Sowerby' s type specimen of the present 

 species contained in that museum. The principal dimensions of Sowerby's 

 specimen are as follows : total length 162 mm., length of body-chamber 65 mm., 

 length of septate part 97 mm., diameter of base of body-chamber 56 mm. (nearly). 

 It is, of course, not possible to make a very close comparison between the type 

 specimen and the one whose dimensions are given in the above description of the 

 species (see PL XV, figs. 2 a, 2 b), as they are both imperfect, the one anteriorly, 

 the other posteriorly; but, judging by the two measurements, there is probably 

 not much difference between them, the proportions being the more easily realised 

 as the specimens are nearly the same size. 



There is a noteworthy agreement in the figures of this species given by different 

 authors in respect to the remai'kably plano-convex form of the shell when the 

 ventro- dorsal profile is looked at (PL XV, fig. 1 c). This shape may not always 

 be equally strongly pronounced, but it asserts itself distinctly enough in most of 

 the figures I have seen, hence it furnishes a good guide for the identification of 

 the species (cf. Phillips, ' Geol. Yorks.,' loc. cit. ; Haughton, 'Journ. Geol. Soc. 

 Dublin,' loc. cit.). 



De Koninck's figure of this species, which he called unaccountably " Gompho- 

 ceras fusiforme," represents a longer and more slender form than Sowerby's; it 

 is a question, therefore, in what category it should be placed. It appears to me 

 to be distinct from P. fusiforme. In the description de Koniuck states that the 

 ventral side is more convex than the dorsal, so that in this point at least there is 

 agreement between the two forms. P. fusiforme is nevertheless clearly a much 



