Spira I 



Sutura I 



angle. 



angle. 



20° 



113° 



part 4] THE EVOLUTION OF LIASSTC &ASTKOPODS. 337 



their appearance, and by the fourth whorl from the apex the orna- 

 mentation tj'pical of the species has been established : that is, 

 three strong spirals on the lower half of the whorl, and four or 

 five spiral threads on the upper half, crossed by very fine curved 

 axials. 



Locality and horizon, — The specimens described are re- 

 corded from the Lower Lias of Ballintoy, and are in the Tate 

 Collection, Museum of Practical Greology, Jermvn Street, London, 

 No. 7965. 



Promathildia ibex (Tate). (Text-fig. 22.) 



Cerithium ibe.v Tate (17, pi. ii, fig- 9; also 20, p. 405 & pi. xxvi, fig. 18). 

 C. ihe.v Richardson (16, pi. xiv, fig. 3). 



Dimensions of a typical specimen. 



Xiengtli. Breadth. Length of spire. 



12 mm, 32 per cent. 75 per cent. 



Slender shells, with slightly heterostrophic embryonic whorls ; 

 the succeeding whorls are angular and quite smooth, except for 

 two spiral bands, one of which forms the anterior carina, while 

 the other maj^ be regarded as a sub-sutural band just beneath the 

 posterior suture of the whorl. Our specimens, though smaller, 



conform with Tate's description 



Fig. 22. — Promathildia ibex, in every respect, except that in 



JEmhryonic tvJiorls X 10 all cases the mouth is circular 



(approximately) sliowing without an}^ suggestion of a canal. 



lieteroBtropliic character. Locality and horizon. — 



Our specimen is from the Lower 

 Lias {valdani zone) of Leckhamp- 

 ton, Cheltenham (L. Richardson 

 coll., L.Gr. 3 a). Another speci- 

 men from the same horizon 

 was found at Hucclecote, near 

 Grloucester (L. Richardson coll., 

 L.Gr. 4). Very near this horizon are found other z^^^'.r-like 

 specimens practically identical with the above, but showing slight 

 variations in detail; for instance, in a specimen from the jamesoni 

 zone of Toddington, near Winchcombe (L. Richardson coll., 

 L.Gr. 22), there is an additional sub-sutural band, almost 

 touching the first-formed. In other forms several additional 

 spirals are added in the later whorls, as, for instance, in a specimen 

 from the Lower Lias of Bengeworth, Worcestershire (Natural 

 History Museum, 0. 10739). 



Promathildia liqatuealis (Tate). (Text-fig. 23, p. 338.) 



Cerithium ligaturale Tate (22, p. 7). 



Dimensions of holotype. 



Length. Breadth. Length of spire. ^^^^^^^ ^mi^u! 



14-5 mm. 28 per cent. 79 per cent. 17° 112° 



