434 Mr. G. C. Crick on some 



of the test. " Wrinlde-layei- " composed of fine, regular, 

 close-set longitudinal lines. 



Di7nensions. 



Diampter of shell 141 mm. (1000) 



Width of uml'ilicus (at suture of shell) 49 „ (347) 



Width of umbilicus (at its margin) .... 63 „ (37-5) 



Heicht of outer whorl 51 „ (361) 



Dittii ahove precedinfr whorl about 30 „ (2r2) 



Thickness of outer whorl 50 „ (35-4) 



Bemarhs. I have seen only one example of this species 

 — the type specimen — which is in the collection of Dr. A. H. 

 Foord, F.G.S., of Dublin. 



An unsuccessful attempt has been made to display the 

 suture-line, the whorl having been ground away too deeply 

 to show the precise form of this important character. 



The " wa-inkle-layer " is particularly well shown on the 

 first portion of the last whorl. 



The trivial name trapezoidah's refers to the form of the 

 transverse section of the whorl of the adult shell. 



Affinities and Differences. This species can be readily dis- 

 tinguished from all the other species of Pericijdus from the 

 Carboniferous Limestone of Ireland that are known to me by 

 the flattened trapezoidal form of the cross-section of its whorls 

 and by their very distinct ornaments, which appear to be 

 continued quite to the aperture of the shell. 



Horizon and Locality. Carboniferous Limestone : St. Dou- 

 lagh's, Co. Dublin, Ireland. 



Pericyclus rotuli/ormis, sp. n. (Fig- 3.) 



Sp. char. Shell discoidal, somewhat compressed, rather 

 rapidly increasing, with a moderately wide umbilicus; 

 greatest thickness at the margin of the umbilicus, ranging 

 from about three eighths to about three sevenths of the 

 diameter of the shell ; height of outer whorl ranging from 

 about two fifths to about three eighths of the diameter of the 

 shell, "VVhorls few (exact number unknown) ; inclusion two 

 thirds ; umbilicus shallow, with slo])ing sides and subaugular 

 margin, ranging from a little less to a little more than one 

 third of the diameter of the shell in width, exposing the edges 

 of all the inner whorls. Wiiorl subcordatc in cross-section, a 

 little higher than wide; indented to about one third of its 

 height by the preceding whorl ; jjeriphery narrowly convex, 

 imperfectly defined, continuous with the sides ; sides feebly 

 convex, somewhat ilattened; umbilical zone fairly well 

 defined, narrow, sloping towards the centre of the umbilicus 



