part 4] 



THE EVOLUTIOI^ OF LIASSIC aASTEOPODS. 



829 



Fig 



A smooth slender form, with convex whorls showing no trace of 



angularity, smooth through- 

 16. — Zygoplem-a nuda (Tate) 

 non Milnster. Diagram sJioiv- 

 incj the curve of the growth-line. 



out, or ornamented by ex- 

 ceptionall}^ fine growth-lines ; 

 with no trace of costse at any 

 stage in development, and 

 no spirals. Mouth incom- 

 plete ; oval. 



This specimen differs from 

 Zygopleura deleta in the 

 rounded whorl and in the 

 greater curve of the growth- 

 lines, and it appears to differ from Turritella nuda Mimster, to 

 which it was referred by Tate, in the absence of spirals. 



Locality and horizon, — The holotype is from the Middle 

 Lias of Huntcliff (Yorkshire). 



Spiral 



Sutuval 



angle. 



angle. 



30° 



105° 



,^, ^.. -Zygopleura capricornu, 

 sp. nov. Holotype X 8. 



Zygopleuea capeicort^'xt, sp. nov. (Text-fig. 17.) 

 Dimensions of holotype. 



Length. breadth. JLength of spire. 



3 mm. 42 per cent. 67 per cent. 



Holotype : A specimen in Mr. L. Ricliardson's collection (L.G. 44). 



Small forms with strong regular costse, resembling in ornament- 

 ation the Capricorn Ammonites with which they are found. The 

 whorls are distinctly convex, and separated by deep sutures. 



Development. — The first 

 Fiof. 17. — Zyg'opleura capricornu, four whorls are quite smooth, 



then strong slightl}'- cui-ved 

 axials (about as wide as the in- 

 terspaces ) appear ; this type of 

 ornamentation is characteristic 

 of the remainder of the shell. 



This little species is charac- 

 terized by the strength and 

 regularity of its costse and its 

 convex whorl, which serves to 

 distinguish it from Zygopleura 

 cf. hlainvillei . 



Locality and horizon. — The holotj^pe is a specimen from 

 the Lower Lias (capricornu s zone) of Stonehouse, near Stroud 

 (Grloucestershire). Specimens which appear to be almost identical 

 have been found by Mr. Richardson near Naunton Park Schools, 

 Cheltenham, at a somewhat lower horizon (valdani zone). Other 

 closely similar specimens were found by Dr. W. D. Lang at 

 Black Ven, Dorset, in the latcecosta sub-zone, 78 feet above the 

 Belemnite Stone (Lang Coll. No. 1030). 



