﻿HUDLESTONIA SINON. 227 



way, were probably not the same as those of Oxynot. oxynotum. It is on these 

 grounds, as I have said, that I create the genus Hudlestonia. 



The broad, shallow, little-denticulated lobes of this genus distinguish it at once 

 from any of the previously described genera ; and this feature separates Hudl. 

 Sinon from either Gramm. aalense, Ludwigia Murchisonse, or any other species which 

 are ornamented with bifurcate radii. The suture-line of this genus, it may be 

 remarked, is decidedly Arietan in character ; and its shallow lobes — which become 

 more shallow as development proceeds — are caused by the extension of the lateral 

 area, producing, as it were, a stretching of the suture-line. 



The British species of Hudlestonia have been derived from one particular 

 horizon, namely, the Jurense-zone ; and in the Cotteswolds they are confined to 

 the Dumo?-tieria-beds. A form from the Opalinum-zone — " upper part of Trigonia- 

 navis-beds or lower part of beds with Am. Murchisonse " * — namely, Am. Fredericii, 

 Branco, which is almost identical with Am. serrodens, is the only species of the 

 genus which I cannot record in this Monograph. 



Hudlestonia Sinon (Bayle). Plate XXXVIII, figs. 13—16. 



1846. Ammonites Mukchison^:, Schmidt (non Soicerby). Petrif.-Buch, pi. xxxv, 



figs. 2 a, b (not 2 c, d). 

 1874. — costula, Dumortier (non Reinecke). Etudes pal. Bassin 



Rhone, iv, pi. li, figs. 1, 2. 

 1878. Ludwigia Sinon, Bayle. Explic. carte geol. France, vol. iv, pt. 1, 



pi. lxxxiii. 

 1885. Hahpoceras Sinon, Haug. Beitr. Monogr. Harpoceras ; Neues Jahrbuck 



fur Mineral, &c, Beil.-Bd. iii, p. 669. 



Discoidal, compressed, carinate. Whorls broad, ornamented with subsig- 

 moidal, ventrally-inclined, primary and secondary radii ; primary radii situated on 

 the inner third of the whorls, and bifurcating or trifurcating to form the secondary 

 radii; radii disappear in adult. Ventral area undefined, slightly carinate on core 

 (no hollow carina ?) ; in adult age no ventral area, the compression of the sides 

 forming a carina. Inner margin sloped, slightly convex. Inclusion about two- 

 thirds. 



One extremely poor cast is all the material I have seen. It is even doubtful 

 if it exactly represent Bayle's species ; but his figure is certainly the nearest form. 

 As this specimen only differs in certain minor points, I could not think of mak- 

 ing a new species on such material ; but the following points should be noticed. 

 Bayle's fig. 1 has a slightly smaller umbilicus than the specimen before me ; its 



1 Branco, " Unt. Dogger Deutsch-Lothringen," ' Abh. z. geol. Spez.-Karte,' Bd. ii, pi. iii, 1879. 



