APHELECERAS HIBERNICUM. 69 



side by a narrowly rounded, not very prominent rim. The sides are flattened in 

 the adult stage, and converge rather rapidly from the umbilical edge towards that 

 of the periphery. 



The apex is bluntly conical and subcircular in section ; the extremity is not 

 seen in any of the specimens examined, but at a point which must be very near it 

 the diameter is 2'5 mm., the ornaments being here quite clearly defined. The 

 whorls begin with what Prof. Hyatt would describe as a gyrocerau curve, which 

 remains free to the extent of fully half a volution before the succeeding one comes 

 in contact with it. 



The septa (here described from a fragment which shows them only in the 

 adolescent stage of the growth of the shell) are numerous, the sutures indicating 

 by their curvature that the chambers were rather deep. Where the whorl has a 

 transverse diameter of 9 ram. the sutures are from 3 mm. to 3'5 ram. apart ; where 

 this diameter has increased to 15 rara. the sutures are from 4 mm. to 5 mra. apart. 

 At this (adolescent) stage of growth there are thirty-two septa in a whorl whose 

 diameter is 55 mm. 



The siphuncle occupies a position slightly above the centre of the septa ; its 

 diameter is not known, as it is only seen at the base of a fragment of the body- 

 cliaraber. 



The ornamentation consists primarily of a series of well-defined longitudinal 

 (or spiral) ridges, which become more widely separated in proportion to the increase 

 iu diameter of the whorls ; these ridges, crossed transversely by fine lines of 

 growth, become thus minutely but distinctly crenulated. The lines of growth 

 make a very deep and narrow, backwardly directed sinus in crossing the periphery, 

 being here very close-set and regular. The longitudinal ridges are most strongly 

 developed upon the sides of the shell, they tend to die off on the dorsal area, and 

 disappear entirely on the completion of the first two whorls, or thereabouts. 



Dimensions. 



Tjpe specliiieii. Large spechueii. 



(Brit. Mus.) (Mus. Sci. and Arr, Dublin.) 



Diameter of shell . . .72 mm. 115 mm. 



,, umbilicus (from edge to edge) 



about . . 42 ,, GO ,, 



,, ,, (from suture to suture) 33 ,, 55 ,, 



Height of outer whori . . . 24 ,, 38 ,, 



Width of periphery of outer whorl . . 10 ,, — 



Thickness of shell at umbilical edo-e about . 21*5 ,, — 



'CD' 



Affinities. — This species is very similar iu many respects to A. mutahile, 

 M'Coy, sp., differing therefrom in the free condition of the first whorl, in the 



