74 DR. L. F. SPATH ON THE AMMONITES [vol. lxxix, 



(E) Genus Arietites Waagen. 



This genus, restricted to the turneri group (not including such 

 forms as, for instance, Ammonites denotatus, tenellus Simpson, 

 A. impendent Young & Bird, which are more nearly allied to 

 Asferoceras), first appears on the Dorset coast at about 36 feet 

 below the fo'rcfo'-nodular bed. Its upward range is not yet deter- 

 mined; but, if we may judge by examples of Arietites collected at 

 distances of 5 to 10 feet above the o irchi -tabular and still close to 

 the true Arietites turneri, the genus passes beyond the limits of 

 the section here described. The specimen from bed 77, closely 

 allied to A. turneri as stated, is the ' Arietites sp.,' included by 

 Mr. S. S. Buckman l in the ' brooki series.' Dr. Lang's specimen 

 from near bed 80 2 is too badly preserved for certain reference to 

 either Asteroceras or Arietites. 



The earliest forms of Arietites, from bed 73, seem to be referable 

 to three new species. One with rather distant costation looks some- 

 what like Ammonites bucklan&i costaries Quenstedt 3 ; another 

 is homoeomorphous with the later true Arietites turneri; and the 

 third has closer costation than the others. The poor preservation 

 of the inner whorls in all three and the absence of suture-lines 

 make it necessary for the present to refer them to new species, on 

 the evidence of external characters and in view of the fact that 

 even in the later Arietites of the brooki bed the inner whorls are 

 very distinct from those of the still later turneri group. It should 

 be pointed out, however, that the inner whorls of the three new 

 species are not markedly smooth ; whereas one specimen at least of 

 the presumably late Arietites plotti (Reynes) has the 'delayed' 

 inner whorls of A. hrooJci. Similarly, in broo7ci-like Asteroceras 

 of a much later date (but until now generally confused with the 

 earlier Arietites) the inner whorls may resemble those of the true 

 A. brooki. 



The real horizon of the last-named species was long a matter of 

 speculation among workers on ammonites, and it can only now be 

 definitely stated, thanks to Dr. W. D. Lang's careful collecting, 

 that this horizon is some 23 feet below the birchi bed and not 

 above it, as had been argued from biological considerations : that is, 

 it is not Tpost-turneri in date, as it was thought to be in develop- 

 ment. The brooki bed is crowded with small Arietites, differing 

 in thickness and in the spacing of the ribs; but this great variability 

 is apparently confined to young individuals, and the adult speci- 

 mens all conform to the typical A. brooki. It may be added 

 that Quenstedt's fig. 7 of pi. xvii {A. scipionianus olifex) may 

 be a true Arietites of this group, whereas his ' Ammonites 

 brooki ft" 1 (pi. xx, figs. 11 & 12) is an Asteroceras. 



It has already been mentioned that no species of Amioceras 



1 Q. J. G. S. vol. lxxiii (1917-18) p. 298. 



2 Proc. Geol. Assoc, vol. xxv (1914) p. 316. 



3 ' Ammoniten des Schwabiscken Jura' 1883, pi. xi, fig 1 . 1. 



