part 4] 



THE EYOLTJTIOlSr OF LIASSIC GASTROPODS. 



327 



Dimensions of our specimen (estimated). 



Length. 

 9 mm. 



JSreadth. 

 30 per cent. 



Length of spire. 

 77 per cent. 



Spiral 



Snfural 



angle. 



angle. 



22° 



111° 



Fig. 14. — Diagram slioiving 

 the adult wliorls of various 

 species of Zygopleura. 



Fairly slender form, whorls almost flat, separated by well-marked 



sutures, ornamented by moderatel}^- 

 fine, slightly-curved, axial ribs. It 

 should be noted that Zygopleura 

 hlainvillei does not include those 

 forms with spii'al ornament that 

 have frequently'' been called Cliem- 

 nitzia hlainvillei, for they are 

 species of Katosira. 



D e V e 1 o p m e n t. — Apical whorls 

 not preserved ; the first whorl pre- 

 sent, probably tlie third, is smooth, 

 and is follow^ed by a whorl orna- 

 mented with simple, straight, axial 

 ribs, not widely separated. This 

 type of ornament persists thi-ough 

 several volutions ; but, as the adult 

 whorls are approached, the ribs tend 

 to become increasingl}^ curved. 



Locality and horizon. — Our 

 sjDecimen is from the Lower Lias 

 {^armatus zone) from the brick- 

 works. Folly Lane. Cheltenham, and 

 is in Mr. L. Richardson's collec- 

 tion (L.Gr. 8). Another specimen 

 in the same collection (L.Gr. 30) 

 from the oxgnotus zone, near 

 liugb}', resembles the preceding 

 very closely ; but the ribs are, if 

 anything, somewhat coarser, and 

 show no tendency to become 

 curved, features which one might 

 expect in a related form from a 

 lower horizon. Forms closely 

 resembling the above were also 

 found in the Lower Liassic claj^s 

 on the tunnel-heaps at Old Dalby 

 (Leicestershire) ; the majority of 

 these, how^ever, are slightly more 

 advanced, for the axial ribs are 



closer together and slightly more curved (text-fig. 14 d) . 



Typical examples of this species have been presented to the 



British Museum of Natural History (G. 24870-72). 



a=Zygopleura sp. (See p. 326.) 



Rhaetic. J. W. Tutcher coll. 



No. 1009. 

 h = Zygopleura sp. (related to c). 



L. Richardson coll. L.G. 30. 

 c= Zygopleura cf. hlainvillei. 



Same collection, L.G. 8. 

 d^ Zygopleura cf. hlainvillei ^ 



J. W. T. coll. No. 1004. 



