part 4] JURASSIC CHEOIS^OLOGT : LIAS. 299 



described as having a keel like a piece of string rolled round. ^ This 

 fits the present species well ; there is a distinct angle at jbhe junc- 

 tion of periphery and keel ; but Quenstedt's is a more slowly coiled 

 form. The present species has the side-view of Blake's ^cjoceras 

 nigrum, but the periphery of Quenstedt's Ammonites miserahilis. 



Before proceeding further it is necessary to mention Hyatt's 

 and Quenstedt's later interpretations of their species. In 1889 

 Hyatt recognized his Fsilocei'cis acutidorsale as an Arnioceras, 

 and identified it as one variety of Ammonites miserahilis, his 

 Arnioceras cuneiforme being another. He then gave figures of 

 these forms, and these figures must be regarded as at least the 

 lectotypes of his original species. But his original description of 

 A. cuneiforme speaks of ' pilae ' on the side, and ' a keel indicated 

 by a ridge ; channels obsolete ' — characters which do not seem to 

 appear in his later figures. In this case, if not in the former, he 

 possibly combined various forms in his two species. 



Quenstedt has also combined various forms in his later work — 

 ' Ammoniten d. Schwabischen Jura ' pi. xiii, figs. 27-30. His 

 figs. 27 & 28 are without distinct keel, his fig. 29 has a distinct 

 keel ; and his fig. 30 from Tewkesbury has, in my opinion, nothing 

 to do with Arnioceras : it is a young stage of Bifericeras.'^ 



A table of the proportions of the various forms discussed may 

 now be given. 



Table XVI. — Peoportions : Arxioceras, Smooth Species. 



Periphery angulate. 



1. Arnioceras cf. acutidorsale (Hyatt). 



{Ammonites miserahilis Quenstedt, siii, 



27)3 j^_26, 22, 20, 60. 



(Am. miserahilis Qwensteclt xiii, 28)^ .. . F. 17-5, 22, 21, 59. 



2. Arnioceras acutidorsale (H.jsitt), a, 4!^ ... F. 32, 26, — , 54. 



3. Arnioceras cuneiforme Hysitt, ii. 5 '^ F. 20'5, 25, 24?, 55. 



4. Arnioceras nigrum (Blake). 



{Mgoceras nigrum Blake, vi, 6) ^ C F. 20-5, 28, — , 49. 



iT.19, 27, 26, — . 



Periphery with distinct keel. 



1. Arnioceras aff. miserahile (Quenstedt). 



(Ammonites miserahilis Quenstedt, pi. 



xiii, fig. 29)3 Y.17, 22, 19, 59. 



2. Arnioceras miserahile (Quenstedt). 



{Amtnonites miserahilis, Quenstedt, pi. 



viii, fig. 7)*^ F.16-5, 25, — , 56. 



3. Ai'nioceras flavum, nov. 



{Arietites semicostatns Wright, i, '7)'' ... F. 28, 26, — , 52. 



(holotype, PL XXXI, fig. 2) S. 25, 27, 20, 51. 



4. Arnioceras Isevissimum (Quenstedt). 



{Ammonites falcaries Isevissimus Quen- 

 stedt, pi. xiii, fig. 18) 3 F.30, 33, 21, 44. 



^ 'Ammoniten d. Schwabischen Jura' p. 71. 



■^ 'Yorkshire Type Ammonites' ii (1913) p. Hi. 3 Op. jam cit. 



-^ ' Gen. Arietidge.' ^ R. Tate & J. F. Blake, ' Yorkshire Lias ' 1876. 



^ Op. jam cit. 1856. '' ' Monogr. Lias Ammonites ' 1878. 



