APHELECERAS MUTABILE. 



65 



narrow furrow dividing the latter from a broad elevation on which there are the 

 remains of two ridges, which have here nearly become obsolete, as is usually the 

 case with the less prominent ornaments in this group of shells. Succeeding the 

 elevation just mentioned is a broad and deep furrow, bounded above by a 

 prominent rounded (in the worn condition of the specimen) ridge ; between this 

 and the peripheral border is another furrow, shallower than the one first described. 

 The sides of the umbilicus descend very steeply to the sutural line of the preceding 

 whorl. The impressed zone appears to have been shallow, but the test is wanting 

 here. The lines of growth, which pursue the usual sigmoidal course transversely 

 to the whorls, are but faintly visible here and there upon the test. 



As the specimen is only a fragment of the body-chamber, neither the septa nor 

 the siphuncle can be seen. 



Affinities. — These are with Stroboceras sulcatum, J. de C. Sow., sp., but the 

 great thickness of the fragment described clearly distinguishes it from that species, 

 though it agrees therewith in the other features observable. 



Dimensions. 



Diameter of shell 



Height of outer whorl (dorso-ventral) 

 Thickness of whorl at umbilical edge 

 Width of periphery where height of whorl 



is 15*5 mm. .... 



Depth of umbilical wall of outer whorl (where 



height of whorl is 15'5 mm.) from edge to suture . 



S. crassiim. S. sulcatum. 



(Sowerby's type, Min. 

 Conch., vi, pi. dlxxi, fig. 2.) 



? mm. 



19 

 17 



25'5 mm. 

 10-25 „ 

 9 „ 



Irish speciineu of 

 S. sulcatum. 



4*5 mm. 



7-5 



These comparative measurements bring out very markedly the much greater 

 thickness of the species under description, when compared with S. sulcatum. 

 Locality. — Ring, near Bnniskillen, county of Fermanagh. 



Genus Apheleceeas, Hyatt, 1883 (emend. 1893). 

 Group of Aphbleceras mutabilb. 



a. Aphelkceras mutabile, F. M'Goy, sp. Plate XXI, figs. 1 — 3. 



1844. Nautilits (DisciTEs) MUTABiLts, J'. ilf'Coy. Synop. Carb. Foss. Ireland, 



p. 18, pi. iii, fig. 7. 

 1860. DisciTKS MUT.vBiLis, R. Oriffith. Journ. Geol. Sue. Dublin, vol. ix, p. 77. 



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