part 4] THE EYOLrTioN or lias sic gasteopods. 319 



stouter. It is ornamented by equally- spaced spirals and axials, 

 which produce sharp denticulations where they cross. The form 

 of the aperture cannot be determined. 



Development (see lig. G a, p. 307). — The upper whorls are not 

 clear in Tate's type-specimen, but in a smaller specimen in the same 

 collection, the only ornament present to the end of the fourth whorl 

 consists of one or more spirals ; and not until the fifth whorl are 

 these crossed by axials. The axials rapidly become strong, and by 

 the sixth whorl the shell has three spirals crossed by axials, pro- 

 ducing a coarse network, with sharp tubercles at the points of 

 intersection. Throughout the remainder of the shell this orna- 

 mentation is retained, but becomes relatively finer as additional 

 spirals are intercalated. On the penultimate whorl, six spirals are 

 present ; and, since the spirals are closer than the axials, an oblong 

 reticulation is produced. The aperture is not clearly visible, but 

 there are some signs of an anterior sinuosity. 



Locality and horizon. — The holotype is a specimen from 

 the Middle Lias of Eston (Yorkshire). 



Pkoceeithium cf. vejS^d^ense Cossmann. 



Small slender forms which appear to be referable to this species 

 in having numerous flat whorls, with a small apical angle not 

 exceeding 20°, and exhibiting the characteristic ornamentation of 

 Dr. Cossmann's figured specimen. Aperture indistinct. 



The material that we have examined includes several varieties : 

 some are strongly costate on the last whorl preserved, others show 

 the catagenesis characteristic of the species (that is, axials are 

 confined to the upper whorls, and the later whorls are ornamented 

 by spirals onl}"). 



These forms may be regarded as coming from a primitive three- 

 spiralled stock ; but, while the forms constituting this groujD have 

 flat whorls separated by deep sutures, they have stronger and more 

 widely- spaced axials than species of the P. ogerieni group. A 

 point of great interest is that these specimens show catagenesis of 

 ornament approximately of the type shown by P. numismale, but 

 the shells exhibit no tendency to assume a pupoid form. 



Locality and horizon. — The numerous specimens which we 

 refer to this group were found by Mr. J. W. Tutcher in Moore's 

 ' Gutter-Bed' {angulata zone), Whitchurch, near Bristol. ^ 



Proceeithium subfistulosum (Tate). (Text-fig. 10, p. 320.) 



Gerithium subfistulosum Tate (22, p. 8). 



Dimensions of specimen presumed to be the holotype 

 (estimated). 



-r -7 -D jj.1 T J.1 jy • Spiral Sutural 



±ienqtli. Isreadth. Length of spire. -^ ■, , 



'^ n -J -tr angle. angle. 



40 mm.? 18 per cent. 85 percent. 16° 98° 



^ C. Moore, ' On Abnormal Conditions of Secondary Deposits, &c.' Q. J. Gr. S. 

 vol. xxiii (1867) p. 497. See also W. Lonsdale, ' On the Oolitic District of 

 Bath ' Trans. Geol. Soc. ser. 2, vol. iii (1832) p. 244. 



2a2 



