52 W. S. Hudkston — On the Yorks/n're Oolites. 



Jurassic Turbos becomes large or small according to the views of 

 particular authors. 



That Amberleya and the so-called LittorincB should be classed 

 under the Turbinidce rather than under the Littorinidce is extremely 

 probable. The title of the smooth forms of Monodonta to be 

 separated from Turbo, because of the columellar tooth, which is 

 very irregularly developed, is also doubtful, since there seems to 

 be a singular connection between the small smooth Turbos of the 

 Lias and the "Monodontas" of the Oolite. It is best to regard 

 Monodonta, Crossostoma, etc., as merely of subgeneric value. 



68. — Turbo (Monodonta) l^vigatus, Sowerby, 1821. Plate II. 

 Figs. 4, 4a, 46 ; 5, 5a; 6, 6a. 



N.B. — In a general sense this should be viewed as a group rather 

 than as one species. The references below must not in every case 

 be regarded in the light of actual synonyms, but rather as represent- 

 ing sections of the group. Hence no chronological arrangement. 

 1821. Nerita Icevigata, Sowerby. Min. Conch, pi. 217, fig. 1. 

 1854. Monodonta Icevigata, Sowerby. Morr. Cat. p. 258. 

 N.D. Nerita bellulata, Bean MS. 

 1843. Trochus Labadyei, D'Archiac, Mem. See. Geol. Fr. vol. v. pt. 2, p. 379, 



pi. 29, fig. 2, 2a. 

 1852. Trochus acmon, D'Orbigny. Terr. Jiirass. p. 278, pi. 314, figs. 1 — 4. 

 1860. Monodonta papilla, Heb. et Desl. Bull. Soc. Linn. Norm. p. 59 of sep. 



copy, pi. 3, fig. 1. 

 1867. Chrijsostoma ovulata, Heb. et Desl. Laube, Gaster. von Balin, p. 13, 

 pi. 3, fig. 3. 



Compare also — 

 1850. Crossostoma discoideum, Morris and Lycett. Gt. Ool. Moll. p. 73, pi. xi. 

 fig- 7. 



Bibliography, etc. — Although specimens of this plentiful and wide- 

 spread group are not extremely rare in the Yorkshire Dogger, yet it 

 would seem that none were available, when Phillips's early editions 

 were published. It is somewhat singular that no notice should have 

 been taken of these fossils in the third edition of the G.Y. (1875), 

 notwithstanding their obvious and unmistakeable difference from the 

 species just described (No. 67). In the Leckenby Collection and 

 also at York they are known as " Nerita bellulata," Bean, MS. 



None of the three Yorkshire forms figured answers exactly to 

 Sowerby's type of Monodonta Icevigata,^ which measures as follows : 

 — Height 12 mm., width 13 mm., spiral angle (approx.) 90°, ratio 

 of body-whorl to entire shell 75 : 100. Hence the type of the 

 M. laevigata group is median between such extremes as Monodonta 

 acmon, D'Orb., with a spiral angle of 65°, and the highly rotelliform 

 section with an inconspicuous spire, which is represented by Mono- 

 donta (Chrysostoma) ovulata, H. and D., and perhaps yet further by 

 Crossostoma of the Great Oolite. In the collections from the I.O. of 

 the South of England the median form is well represented, but 

 graduates into the rotelliform section ; whilst the papilliform section 

 is more sharply separated. Thus Mr. Tawney (Dundry Gastero- 



1 Kindly lent to me by the Council of the Bristol Museum at the instance of 

 Edward Wilson, Esq., F.G.S., the Curator. 



