430 Mr. Gr. C. Crick on some 



(Natural History),' with only a few alterations. The term 

 "umbilical zone"* has been substituted for the somewhat 

 ambiguous expression " inner area of whorl " ; and since in 

 some species this zone slopes towards the centre of the umbili- 

 cus 1 have thought it advisable in the measurements of these 

 species to give the width of the umbilicus both at its margin 

 and at the suture of the shell. The term " peristome " is 

 used for the margin of the aperture, and in describing the 

 aperture Prof. Hyatt's terms have been adopted — " crest " for 

 projecting parts and " sinus " for inflections of the outline ; 

 also the same author's term " hyponomic sinus " for the ventral 

 sinus of the aperture and of the lines of growth, it being due 

 to the hyponome or motor organ, usually called the " fleshy 

 funnel " in the recent Nautilus *. 



Excepting figs. 6 and 15 and the drawings of the suture- 

 lines, I have to thank Dr. Foord for the original drawings of 

 the illustrations accompanying this paper. 



Pericyclus Foordi, sp. n. (Fig. 1.) 



Sp. char. Shell discoidal, somewhat compressed and rather 

 widely umbilicated ; greatest thickness at the margin of the 

 umbilicus, rather more than two fifths of the diameter of the 

 shell ; height of outer whorl a little more than three sevenths 

 of the diameter of the shell. Whorls eight or nine; inclusion 

 fully three fourths ; umbilicus rather deep, displaying the 

 umbilical margins of all the inner whorls, about three tenths 

 of the diameter of the shell in width. Whorl semielliptical 

 in cross-section, a little higher than wide; indented to about 

 two filths of its height by the preceding whorl ; periphery 

 convex, imperfectly defined ; sides feebly convex ; umbilical 

 zone well-defined, sloping towards the umbilicus, and making 

 an obtuse angle with the sides, rather narrow. Body- 

 chamber not fully seen, but occupying at least one half of the 

 outer whorl; aperture not seen, but the peristome probably 

 (judging by the ornaments and the lines of growth) with a 

 feeble lateral crest at about the middle of the lateral area and 

 a deep and wide hyponomic sinus. Depth of chambers not 

 seen; suture-line only imperfectly known. Test ornamented 

 with fine, backwardly directed, and somewhat irregularly 

 riblets, which form a rather low crest at about the 



* See A. Hyatt, " Phytogeny of an acquired Characteristic," Proc. 

 Amer. Philos. "Soc. vol. xxxii. no. 143, pp. 422 et seqq. ; and " Carbon- 

 iferous Cephalopods," 4th Ann. Rep. Geol. Surv. Texas for 1892, pp. 380 

 et seqq. (1893). 



