PERICYCLUS BA1LYI. 147 



Pericyclus Bailyi, G. G. Grid. Plate XL, figs. 1—3. 



1899. Pericyclus Bailyi, G. C. Crick. Aim. Mag. Nat. Hist., ser. 7, vol. iii, 



p. 438, dg. 6. 



Description. — Shell discoidal, somewhat compressed, and rather widely umbili- 

 cated ; greatest thickness close to the umbilical margin, about four-ninths of the 

 diameter of the shell, height of outer whorl about three-sevenths of that diameter. 

 Whorls possibly six or seven, but in default of perfect adult specimens the number 

 must for the present remain doubtful. Inclusion fully three-fourths. Umbilicus 

 about four-elevenths of the diameter of the shell in width, very deep, exposing all 

 the inner whorls, the margin subangular, sloping inwards, steep. Whorl semi- 

 elliptical in cross-section, slightly wider than high, indented to about three-eighths 

 of its height by the preceding whorl; sides very slightly convex, merging in the 

 rounded periphery. 



Body-chamber occupying the whole of the last whorl. 



Suture-line as in PL XL, fig. 3. 



Test ornamented with fairly regular, not very prominent ribs, which are stronger 

 in the young shell than in the adult, in which they become almost obsolete ; they 

 form a gentle, backwardly directed curve on the sides of the shell, and are sharply 

 bent backwards upon the periphery. Constrictions appear at intervals, about six 

 being counted in a whorl ; their direction corresponds exactly with that of the ribs. 



Dimensions. 



Type specimen in Specimen from St. Dou- 

 tlie British lagh's, in the Wood- 

 Museum wanlian Museum, Cam- 

 (No. C. 298). bridge (No. 439). 



t mm. . oo mm. 



Diameter of shell (body-chamber, incom- 

 plete in Cambridge specimen) 



„ umbilicus (vi\i>;e to v(\^v) . 35 „ . 20 „ 



,, ,, (suture to suture) . 30 ,, . 17 ,, 



Height of outer whorl . . 41 „ . 24 „ 



,, above preceding whorl (about) 26 .. . — ,, 



Thickness at umbilical margin (about) . 44 „ . 2D „ 



Affinities. — The relations of this species to Pericyclus Foordi and to /'. rotuli- 

 formis are referred to under the descriptions of these species. 1 The great depth 

 of the umbilicus and the comparative feebleness of the ribbing, together with the 

 rounded form of the periphery, are its distinguishing features. 



Remarks— -As regards the form of the shell and the character of the ornaments, 



i P. Foordi, p. 141 ; l>. rotuliformis, \>. 148. 



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