46 CARBONIFEROUS CEPHALOPODA OE IRELAND. 



The test is quite smootli. 



Affinities. — This species is neav\j related to Poterioceras fusiforme, J. Sow., 

 sp., from wliich it is readily distinguisbed by its wider and less oblique septa, and 

 by its being almost equally inflated on the ventral and dorsal areas. 



Bemads. — None of the specimens of this well-marked species have been up to 

 the present time obtained at St. Doulagh's, and it is remarkable that many of the 

 species obtained there differ from those yielded by the Clane quarries, less than 

 twenty-five miles distant. This is especially the case with the Cephalopoda. 



Localities. — Clane, county of Kildare; county of Limerick (exact locality 

 unknown). 



POTEKIOCERAS VENTRICOSUM, M'Col/. Plate XVII. 



? 1S43. Oetuocekas latissimdm, J. E. Porllock. Eep. on the Geol. of Londou- 



derry, p. 390, pi. xxxv, figs. 2 a, b. 

 1844. PoTEEioCERAS VEKTEicosuM, M'Col/. Svnop. Carb. Foss. Irelaud, p. 10, 



pi. i, fig. 2. 

 1888. — coitDiFORME (pars), A. U. Foord. Cat. Foss. Ceph. British 



Museum, pt. 1, p. 2G0. 



Description. — Shell large, broadly fusiform, much inflated in the upper half, 

 curved in the apical part, the most inflated part being ventro-dorsal as in P. 

 fusiforme. The body-chamber, which comprises the most inflated part of the 

 shell, contracts towards the aperture ; the base is markedly oblique, conforming 

 in this respect to the chambers which precede it. The chambers, as indicated by 

 the suture-lines, are very shallow, the distance between them not exceeding 8 mm. 

 where the diameter of the shell varies from 50 mm. to 70 mm. (PI. XVII, fig. 2 a), 

 thus showing a remarkable uniformity of spacing. The same uniformity is found 

 in another somewhat larger specimen, so that it uiay be taken as a characteristic 

 feature of this species. The section in the young shell is slightly elliptical, the 

 ratio of the two diameters being as 30 : 27. The siphuncle in the young shell is 

 situated at about one-third of the distance across the longer diameter (PI. XVII, 

 fig. 2 6); M'Coy describes it as "large, inflated, and slightly excentric," which 

 probably refers to its position in the adult; if so, it gradually assumes a nearly 

 central position as in some other cephalopod genera. 



The surface of the test is beautifully ornamented with a series of faint 

 longitudinal ridges, easily seen when the light falls upon them from the side ; 

 they may also be felt (PI. XVII, fig. 1 a). These ridges or bauds are crossed by 

 very numerous delicate lines of growth, with stronger ones at intervals. 



