part 4] jUEAssic cheonologt : lias. 315 



Radstock, but Quenstedt figures an approximate form. A. anage- 

 neticum is a Dorset shell : something similar is figured by Hyatt 

 from Semur^; but nothing like either is figured from Wiirtemberg. 



The distribution of the smooth forms is interesting. Yorkshire 

 j'-ields an angulate species and Dorset a earinate one, while no 

 smooth forms are recorded from the Cheltenham area. Neither 

 of the English smooth forms is shown for Wiirtemberg nor Semur : 

 both, yield less advanced angulate and earinate forms, and the 

 former a more advanced earinate, see p. 299. The Rhone Basin 

 seems to lack smooth forms altogether. 



It is for future work to determine, on the one hand, whether 

 such differences are due to incompleteness of records and collections 

 or to want of discrimination ; on the other hand, whether they are 

 due to chronological (strati graphical) or geographical causes. 



Arnioceras fortunatum is interesting for its size. Now bigness 

 seems to be a character of the ammonites of the Radstock district 

 at several dates of the Lias. Roughly speaking, much the largest 

 British examples of Arnioceras, the Oxynotoids, Echioceras, 

 Deroceras, JPhricodoceras, and Uptonia are all from the Radstock 

 district, and yet its remarkable ammonite fauna is almost unknown 

 in British literature. To these ma.y be added, froin the neigh- 

 bourhood, Vermiceras 18 m. (Wright, Monogr. p. 273, Arietites 

 conyhearei)^ the most massive Coroniceras (C. huchlandi), the 

 most bulky JSIicroderoceras, the largest Arietites. In some cases 

 this phenomenon of local bigness is due to the presence in the 

 district of special strata which have presumably been destroj^ed in 

 other areas; but is this the whole explanation ? 



I conclude this palseontological survey with the impression that 

 the principal need of British Jurassic stratigraphy at present is 

 palaeontological work — the figuring of the large amount of material 

 Avhich has accumulated. 



[d] Synopsis and Index Details. 



Below is a Synopsis of newly-described or named genera and 

 species — new names being in heavy type. The index details 

 are given Avith the following contractions : — 



T.d. Type- description, original description (protolog-). 



T./. Type (original) figure (protograph). 



'Som,. When and where the present trivial name was given. 



TX. Locality which furnished the type. 



Kor. Geological position — in words of original description, or 

 label. 



Dafe, Age and hemera (r/) as accurately as can be surmised. 



Coll. In whose collection when described, or present resting- 

 place, if kno-wTi. 



Form. Proportional formula — that which seems most representa- 

 tive, if that of more than one size of holotype has been 

 given. 



^ ' Gen. Ariet.' 1889, pi. ii. fig. 10, Arnioceros semicostatum Hyatt. 



