HYDATINA UNDULATA. 99 



convex, flat, or concave — no sinus round the suture, effuse at the base. AVitli 

 regard to the "smooth" character, it is to be noted that one at least — IF. velum — 

 among' living species has longitudinal risings which are interrupted at the colour 

 bands. 



The records of Hydatina are all under the name of Bulla, and one species only 

 has been recorded from the Cornbrash : 



*Bulla undulata (6, 14, 21, 28, 36, 43, 50, 68). 



Hydatina undulata (Bean). Plate IX, fig. 6. 



1839. Bulla undulata, Bean, Ann. Nat, Hist, [2], vol. iii, p. 61, fig. 22. 



1885. — — Hudleston, Gteol. Mag. [3], vol. ii, p. 234, pi. v, fig. 10. 



? 1850. — Morris and Lycett, Great Ool. Mollusca (Pal. Soc), p. 96, pi. viii, fig. 8. 



? 1895. Paheohydatina undulata, Cossraan, Cont. Pal. France, vol. v, mem. 14. 



Type. — " Shell oval, approaching to globular, longitudinally wrinkled or un- 

 dulated, aperture large, comprising nearly the whole shell, but much wider at the 

 lower than the upper part. Apex umbilicated. Length l^in. [30 mm.]. Breadth 

 1 in." [23 mm.] From the Cornbrash of Scarborough. 



Description. — The last whorl in the type specimen rises higher than the pen- 

 ultimate, and therefore forms an elevated ridge round the sunken spire. The inner 

 slopes of this rising meet the penultimate at a considerable distance from the 

 centre and thus must leave the apex exposed, and the same is the case with all the 

 Scarborough specimens — though the size of the exposed apex varies — the speci- 

 men figured by Hudleston having the smallest. The expansion of the aperture and 

 the proportional breadth also vary, the type being one of .the narrowest. The 

 surface of the shell, as indicated by the name, has irregular longitudinal folds, 

 which can scarcely be looked upon as lines of growth, for at about a third down 

 their length they are wrinkled into backwardly pointing chevrons, making a spiral 

 band, and look at first sight accidental. When we consider, however, that in the 

 recent H. velum the longitudinal risings are interrupted at the colour bands, the 

 connection between the two phenomena is indicated. Looking, therefore, again at 

 other specimens of so-called Bulla, we may find indications in all well-preserved 

 ones, here and there, of similar spiral bands of crinkles, which we conclude must 

 be connected with the original colour bands, thus showing that we are dealing with 

 Hydatina (see Morris and Lycett's figure). It is interesting to learn that the 

 lingual ribbon of recent Hydatinse is of a less developed type than that of Bulla. I 

 can see, however, no reason for separating the Jurassic species from the rest under 

 the title Pal&ohydatina unless //. velum is included. 



Relations. — Compared with the fossil Hull hi ;v of other localities and horizons, 



