432 Mr. G. C. Crick on some 



am able to give also the following measurements at different 

 diameters : — 



Diameter of shell .. 79-5 mm. (100-0) oG-Omm. (1000) 40-0 mm. (1000) 



Width of umbilicus 



(at suture ol shell) 25-0 „ (32-0) 21-0 „ (Sro) 160 „ (400) 



Width of umbilicus 



(at its uiargin) . . 3.3-5 „ (42-1) 260 „ (46 4) 220 „ (oo-O) 



Heijjht of outer 

 whorl 



Ditto above preced- 

 ing: whorl 



Thickness of shell . . 43 „ (o4-0) 33-o „ (59-8) 295 „ (73-7) 



Remarhs. I have seen only two examples of this species; 

 they are both in the collection of Dr. A. H. Foord, F.G.S., 

 of Dublin ; their dimensions are given above. The larger 

 specimen is broken transversely across, so as to display the 

 inner Avhorls ; the innermost whorls are much wider than 

 high, but as the shell grows the whorl increases in height 

 more rapidly than in width. This is very apparent from the 

 dimensions of the inner whorls that are given above. 



I have much pleasure in naming the species after 

 Dr. A. H. Foord, who has already made known many new 

 Cephalopoda from the (carboniferous Limestone of Ireland. 



Affinities and Differences. This species can be readily dis- 

 tinguished from Pericijclus trapezoidalis by its feeble 

 ornaments, its relatively more inflated whorls, and the sloping 

 umbilical zone of its whorls; and from Pericydus rotuU- 

 formis* by its less distinct ornaments and the absence of 

 pronounced constrictions. 



Horizon and Locality. Both specimens are from the Car- 

 boniferous Limestone of St. Doulagh's, Co. Dublin, Ireland. 



Pericyc^us trapezoidalis^ sp. n. (Fig. 2.) 



Sp. char. Discoidal, flattened, rather widely umbilicated ; 

 greatest thickness at the margin of the umbilicus, nearly four 

 elevenths of the diameter of the shell ; height of outer whorl 

 about four elevenths of the diameter of the shell. Whorls 

 fairly numerous (exact number not known) ; inclusion rather 

 more than one half; umbilicus shallow, displaying the edges 

 of all the inner whorls, about three eighths of the diameter of 

 the shell in width, with subanguhir margin and nearly vertical 

 sides. Whorl subtrapezoidal in cross section, about as high 

 as wide; indented to about one fourth of its height l)y the 

 preceding whorl ; periphery narrowly convex, imperfectly 



* See infra, p. 434. 



