248 W. R. Rudleston—On the Yorkshire Oolites. 



The " Trochtjs " bisektijs Group. 



There is considerable difficulty in dealing with the rare and beau- 

 tiful fossils which make up the above-named group (see Figures 9, 

 10, and 11). In a general way Phillips's " Troclms" hisertns (G.Y. 

 plate xi. fig. 27) is representative of it ; hence there would be no 

 uncertainty in accepting this name, provided we knew to which 

 form it is more especially applicable. The type was provided by 

 Williamson, but I have never seen it. Bean recognized a species, 

 evidently belonging to the same group, which found admittance, 

 though without either figure or description, in Phillips's first edition 

 (p. 129), as Turbo unicarinatus, Bean MS., and which in the third 

 edition is recorded as Littorina unicarinata, Bean MS. 



As a result of an examination of the collections, it appears to me 

 that there are two forms which may be fairly separable as species, 

 or very marked varieties : each of them is small, under 20 millimetres 

 in height (compare Figs. 96 and 106), but each of them is repre- 

 sented by a megalomorph, occurring as a larger and more vigorous 

 variety. If we are to lump the whole under one designation, 

 " Trochus " bisertus is the correct one ; and where specimens are 

 imperfectly preserved, showing only the trochiforra spire, this is by 

 far the safest plan. Yet it is equally certain that we are not dealing 

 with a Trochus in any of these specimens, but with shells belonging 

 to the so-called Littorines, and which may belong either to Littorina 

 or to a section of Turbo. Adopting names already in use, but 

 without asserting that I am using them in the precise sense intended 

 by the authors, I put forward Littorina biserta, Phil., with its 

 megalomorph Amberleya biserta, and Littorina unicarinata, Bean, 

 which is the commoner form of the two, and is also represented by 

 a large variety, which I believe to have been the original Trochus 

 bisertus of Phillips. This latter is not figured, as I have only quite 

 lately obtained a specimen. 



55. — Littorina (Trochus) biserta, Phillips, 1829. Plate VIII. 



Figs. 9a, b, c. 



1829 and 1835. Trochus buerfus, Phillips, G. Y. p. 129, pi. xi. fig. 27. 

 1854. Trochus Usei-tus, Phil. Morr. Cat. p. 281. 



Compare also, both for this and for Littorina unicarinata. 



Turho generalis. M. Goldf. Petrefact. iii. 92, pi. 194, 4. 

 Turbo suhangulatus, M. Goldf. Petrefact. iii. 92, pi. 194, 5. 



Bibliography, etc. — D'Oi'bigny, who generally quotes Phillips's 

 species, omits this one from the list of Trochus in the etage Bajocien, 

 nor am I aware that the species is in any way referred to in the 

 Prodrome, either under a synonym or otherwise. Neither can I 

 identify it amongst the numerous small trochiform shells figured in 

 the Pal. Fran9. Turbo generalis, M., from the Lower Oolites of the 

 neighbourhood of Amberg, has some slight resemblance. 



Description. — Specimen from the Dogger (zone 1), Peak (Blue 

 Wyke). Leckenby Collection. 



Length 19 millimetres. 



Height of body-whorl to entire shell 68 : 100. 



Spiral angle 60°. 



