part 4] THE EVOLUTIOX OF LIASSIC GASTROPODS. 333 



Development. — The first two and a half whorls are smooth, 

 then strong and practically straight axial ribs appear, numbering 

 about seven to a whorl ; this number increases to about twenty as 

 the adult whorls are approached. It is difficult to discern when 

 the spiral lines appear, but they are clearly developed before the 

 seventh whorl. The median spirals of each whorl are much fainter 

 than those situated nearer the posterior and anterior sutures. The 

 early whorls are tiat or onlj'^ slightly convex, the later whorls are 

 more convex, but not markedly so. 



This species may be taken as a central form in a very abundant 

 and very widely distributed group of species of Katosi7'a which 

 are found in the Charmouthian of this country. Numerous other 

 forms differing only slightl}^ from this may be distinguished ; but 

 it does not appear necessary at this stage to indicate new names 

 for them. Among species named by other authors referable to 

 this group may be mentioned Katosira periniana (A. d'Orbigny), 

 K. cliartroni Cossmann, and K. corvalliaiia (A. d'Orbign}'^). In 

 this country these forms have almost alwaj^'s been referred to 

 Cliemn'dzia hlainvillei (Miinster) ; but Miinster's figure shows 

 tliat his species is without spiral ornament. 



Locality and horizon. — -The specimen figured is from the 

 Lower Lias {jamesoni zone) of Toddington, Winchcombe, 

 (xloucestershire (L. Kichardson coll. L.Gr. 28). Specimens from 

 a somewhat lower hori./.on (hircJ/i zone) at Pebworth (L. Richard- 

 son coll. L G. 61) differ in having stronger, straighter, and more 

 widely-spaced ribs than K. cf. youngi. and probably represent a 

 somewhat more primitiv^e form ; while a slender form resembling 

 the above, but having finer and more distinctly curved ribs, found 

 on the tunnel-heaps at Old Dalby (Leicestershire), is probably a 

 representative from a higher zone. 



Katosira transgressa, sp. no v. (Text-fig. 21, p. 834.) 

 Dimensions of holotype. 



Length. Breadth. Jjength of spire. 



10 mm. 27 per cent. 70 per cent. 



Holotj^pe : A specimen in Mr. L. Richardson's collection, L.G. 25. 



A slender form, identical with A", cf . ijouiigi in the early whorls, 

 but with spirals so accentuated on the last whorl as to produce 

 minute denticulations. The specimen is slightly crushed, and the 

 shape of the mouth is obscure. 



Development. — The first two whorls are smooth, after whicli 

 ribs aj)pear. For the first eight whorls the ribs simply increase 

 steadily in number, and the s])irals are accentuated near the 

 sutures (fig. 21«) exactly as in K. cf. young L On the two whorls 

 before the last the ribs become soinewhat more curved, while on 



Q. J. G. S. No. 308. 2 r 



Spiral 



Natural 



angle. 



angle. 



20° 



106° 



