70 DE. L. F. SPATH ON THE AMMONITES [vol. lxxix,. 



the forms of this group of Arnioceras can he identified with other 

 known species. One new form somewhat resembles Reynes's 

 A. geometricus Phillips, var. hartmanni Oppel (' Monographie des- 

 Ammonites' pi. xv, fig. 19) l but has flattened lateral areas, 

 a rectangular whorl-section and closer costation. Arnioceras 

 hartmanni var. plicata Fucini 2 has its closer costation, but 

 different periphery and suture-line. This new form is transitional 

 to another resembling Ammonites patti Dumortier, 3 with still 

 flatter periphery ; on the other hand, Arnioceras hartmanni, with 

 angular periphery, leads to smooth forms, comparable to A. nigrum 

 (Blake), and having body-chambers at small diameters. This 

 group of forms is thus fairly homogeneous, and the suture-lines, 

 all of the type of that of A. semicostatum (Young & Bird), 

 have short and comparatively wide lobes and saddles, like A. 

 d'Orbigny's and R^nes's (exceptional) figures. It is to be noted, 

 however, that in, for instance, an example of A. semicostatum 

 from Yorkshire, the two types of Arnioceras suture-lines (namely, 

 one having a deep and narrow first lateral lobe ending in two long 

 prongs, and the other a shallow wide principal lobe) occur, not 

 only on consecutive septa, but on opposite sides of the same 

 septum, and therefore care has to be exercised in the examination 

 of suture-lines. 



The examples of Arnioceras from the beds below the brooki- 

 horizon down to bed 72 are hardly identifiable specifically ; but in 

 beds 72 & 71 forms of Arnioceras possessing smooth inner whorls 

 are dominant. They are, with some hesitation, compared to 

 '■A. semicostatum', and certain examples resemble the form with 

 oblique costse that Quenstedt, 4 in his latest work, erroneously 

 identified with his species Ammonites ceratitoides of 1849. 5 Of 

 Quenstedt's later Arnioceras, only the Oil-Shale form A. falcaries 

 olifex ('Ammoniten des Schwabischen Jura' 1883, pi. xviii, fig. 10) 

 can have any affinity with the types here described; but this species 

 is incompletely known. Numerous small forms remain smooth to 

 the diameter of Arnioceras Jlavum S. Buckman, 6 which species 

 was described as having the periphery of Quenstedt's Ammonites 

 miserabilis. But this, apparently, is an error ; for Quenstedt 

 did not describe his A. miserabilis as 'having a keel like a piece 

 of string rolled round,' although on p. 71 of his ' Jura' (not his 

 ' Ammoniten des Schwabischen Jura ') he says that it looks like 

 coiled string, obviously in respect of its very evolute character. 



1 In the text (p. 4) Reynes considered A. liartmanni Oppel to represent 

 the young 1 stage of ' A. geometricus Phillips,' and put it in the ' zone a Ammo- 

 nites multico status.' 



2 Pal. Ital. vol. viii (1902) p. 198 [158] & pi. xxvi [xxix], fig. 12. 



3 ' Etudes Paleontologiques sur les Depots Jurassiques du Bassin du 

 Rhone' vol. ii (1867) p. 119 & pi. xxi, figs. 16-17. 



4 'Ammoniten des Schwabischen Jura' 1883, pi. xiii, fig. 8. 



5 ' Petrefactenkunde Deutschlands, pt. i : Die Cephalopoden ' p. 239 & 

 pi. xix, fig. 13. 



6 ' Jurassic Chronology : I— Lias ' Q. J. G. S. vol. lxxiii (1917-18) p. 298 & 

 pi. xxxi, fig. 2. 



