430 ^rv- G-. C. Crick on fiome 



(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. Tlie term " peristome " is 

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

 aperture Frof. 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, 1 have to thank Dr. Foord for the original drawings of 

 the illustrations accompanying this paper. 



Pericydus 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 filths 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 wiiorls, about three tenths 



of the diameter of the shell in width. Whorl semielliptical 



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



two fiiths 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- 



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



outer whorl j 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. IVst ornamented 



with fine, backwardly directed, and somewhat irregularly 



spaced riblets^ which form a rather low crest at about the 



• See A. Hyatt, " Phylofiony of an acquired Characteristic," Pjoc. 

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

 iferous Cepbalopods," 4th Ann. Kep. Geol. Surv. Texas for 1802, pp.380 

 et seqq. (1893). 



