part 1] OF THE SHALES-WITH-' BEEF.' 69 



Hehl in Zieten, generally included in Arnioceras, the genus 

 Arnioceratoides was proposed, this development being more nearly 

 allied to Coroniceras ; but with only the material that is at 

 present at my disposal, the separation of what here are called true 

 Arnioceras from ' JEpamioceras ' is practically impossible, and so 

 they are all left in Arnioceras sensu lato. 



Unfortunately, Mr. Buckman l in 1911 chose as genolectotype of 

 Arnioceras a form (A. ceratitoides Agassiz=^4. ceras in Hyatt, 

 * Genesis of the Arietidse ' pi. ii, fig. 20) that bas nothing to do 

 with the true A. ceras (Griebel) Hauer (1856) or the earlier (1849) 

 A. ceratitoides Quenstedt, which probably is a b idly-figured 

 example of the same common Adneth form. The suture-line of 

 the misidentilied form of Hyatt agrees neither with that of the 

 ■early Arnioceras ceras, nor with that of typical ' Eparnioceras ' ; 

 moreover, Hyatt's error had been detected already in 1902 by 

 A. Fucini, 3 who included Hyatt's form in Parona's A. dimorplium, 

 although coiling and suture-line differ considerably. The numerous 

 species of the large (probably polyphyletic) genus Arnioceras, 

 require revision ; but a point that may here be made is that, until 

 corrected by the collector, the palaeontologist considered Arnioceras 

 to be one of the most easily- and safely-determinable genera of 

 ammonites, and the reference of, for example, the early Arnioceras 

 acuticarinatum (Simpson) to the late semicostatum zone seemed 

 natural. Another important point is that the dissimilarity of the 

 Arnioceras faunas or, rather, of the common museum-specimens 

 of Arnioceras, from Dorset to Yorkshire, is due to differences in 

 their dates of existence, and probably not to differences in geo- 

 graphical distribution ; and it is the great merit of Mr. S. S. 

 Buckman to have been the first to drive home this lesson of dis- 

 similar faunas. 



The range of Arnioceras, however, through the 53 feet of 

 deposits here mentioned is not uninterrupted, and from several 

 horizons (for instance, 62 & 63) no example of Arnioceras has 

 yet been found, although the absence is doubtless due only to 

 collection-failure. On the other hand, the broolci nodules (74 d) 

 are crowded with Arietites and Cymbites, but no Arnioceras occurs 

 in them ; whereas in the following horizons (74 e & 74 f ) Arnio- 

 ceras is again the dominant genus. 



This last horizon, the Black Arnioceras Bed (74 f) is here 

 denominated the horizon of A. hartmanni (Oppel). Owing to 

 the fact, however, that A. d'Orbigny's 3 figure of Ammonites 

 kridion (renamed A. hartmanni by Oppel), like the Dorset form 

 here described, shows not a bipartite, but a tripartite, external 

 saddle, and that the species has so frequently been misidentified. 

 the name is perhaps not a good one. On the other hand, none of 



1 ' Yorkshire Type- Ammonites ' vol. i (1911) p. vi. 



2 ' Cefalopodi Liassici del Monte di Cetona ' pt. ii, Palasontographia 

 Italica, toI. viii (1902) p. 190. 



:< ' Palt'ontologie Fran^aiee : Terrains Jurassiques ' 1844, pi. Ii, fig's. 1-2, 

 ■&, 4-6, non fig. 3. 



