POTERIOCERAS VENTRICOSUM. 47 



Size. — Length of the most perfect of the specimens figured (fig. 2 a ; wanting 

 the apical part), 170 mm.; greatest diameter 110 mm.; least 30 mm. A larger 

 specimen (a cast) collected by myself and now in the Museum of Science and Art, 

 Dublin, has the following dimensions : length 210 mm. ; greatest diameter (body- 

 chamber) 120 mm. ; least about 20 mm. This specimen is imperfect at both 

 ends, though considerably more than half of the body-chamber remains, as is 

 indicated by its contraction above the inflated part showing proximity to the 

 aperture. 



Affinities. — The species most nearly related to the present one is undoubtedly 

 P. cordiforme, J. Sow., a very large species found in the Red Sandstone Group of 

 the Calciferous Sandstone, at Closeburn, Dumfriesshire. I have, in fact, in the 

 ' Catalogue of Fossil Cephalopoda, British Museum,' 1888, Part 1, p. 260, made 

 M' Coy's species a synonym of Sowerby's, being at that time unable to find 

 adequate grounds for their separation. With better material at my disposal I 

 now deem it advisable to keep them apart, because, in addition to the ornamenta- 

 tion described above, there is a slight but distinct curvature in P. ventricosum in 

 the young shell ; this maybe seen in both the specimens I have figured. This 

 may be better realised by extending the outline of the apical end of the figures 

 until the lines thus drawn meet together; a very perceptible curvature is the 

 result. 



Remarks. — Though only a small fragment, consisting of about six chambers, 

 Portlock's species, Orthoceras latissimum, is difficult to distinguish from M'Coy' s : 

 the septa are equally distant in the two forms, and the position of the siphuncle, 

 a minor consideration here, is apparently also the same. Portlock's specimen, 

 which is labelled " Kildare" (meaning probably Clane, which is in the county of 

 Kildare), is still to be seen in the Museum of Science and Art, Dublin, having 

 survived the vicissitudes through which the " types " figured by Portlock and 

 M'Coy have passed before reaching their present resting-place. 



It is to be regretted that M'Coy should not have referred in his description of 

 P. ventricosum either to Portlock's or even to Sowerby's species. 

 Locality. — Clane, county of Kildare. 



The genus Poterioceras, with which the uncoiled forms of Cephalopod shells 

 terminate in this memoir, has a wide stratigraphical range, extending from the 

 Ordovician to the Carboniferous. Though the first species described was a Carbon- 

 iferous one (" Orthocera" cordiformis, J. Sowerby, ' Min. Conch.,' vol. iii, 1821), 

 the genus originated, as stated above, in rocks of Ordovician age. Under the 



8 



