8. 8. Buchnan — On Jurassic AmmonUes. 357 



the sequence of a long history, a last page in a many-chaptered 

 volume. Its position and character are, in the main, determined by 

 that of the Loess, and it is merely that deposit rich in humus, 

 resulting from the decomposition, through long ages, of many 

 generations of grasses and steppe plants. Not only has it not resulted 

 from changes of a complex, sudden, and variable character, but it 

 is actually being produced at the present day, a layer several inches 

 thick covering some of the ancient Tartar tumuli so frequent on the 

 Eussian steppe. 



Further, there is no proof of the former existence of trees on 

 these areas ; in fact there is actual evidence forthcoming against such 

 a view, both in the pages of history and in the facts of agricultural 

 development. The winds that sweep the steppes, and carry far 

 and wide the loose powdery soil, are directly opposed to the growth 

 of wood or forest on these plains. The Russo-Grerman colonist finds 

 it necessary, when attempting to start a plantation, to grow the 

 trees close together, so that the outer ring may protect the inner 

 ones from the blast. 



Circumstances have been mentioned which tend to show the 

 necessity of overcoming special obstacles ere tree-life can be 

 established, and also proving that it is only invading the steppe- 

 lands along lines where denudation influences are active. The 

 variations in richness of humus may be largely due to climate, and 

 the alteration in thickness to the action of the wind ; but in any case 

 the Black Earth would appear to be a typical surface formation, 

 resulting from the direct chemical combination of organic materials 

 with the soil constituents. 



IV. — Jurassic Ammonites: On the Genus Cfmbites (Neumayr).^ 

 By S. S. BucKMAN, F.G.S. 



THIS paper deals with the species which are considered to belong 

 to the genus Cymhites, and the reason for placing Paroniceras, 

 Bonarelli, as a synonym. A few notes are added concerning Agassi- 

 ceras, which is also affected ; and a description of the little-known 

 species Agassiceras Colesi (J. Buckm.) is given. 



Genus, Cymbites, Neumayr, 1878. 



(Type, Cymbites globosus, Zieten, sp.) 



1875. Agassiceras, Hyatt, New Genera Amm. ; Proc. Boston N. H. Soc. vol. 17, 



p. 225 {pars). 

 1878. CijmUtes, Neumayr, TJnverm. auftret. Cepli. ; k.k. geol. Reichsanstalt. Bd. 



28, Heft 1, p. 64 [28], footnote. 

 1884. Agassiceras, Zittel, Palseont. Bd. i. Abth. 2, Lief. iii. p. 455. 

 1884. Cymbites, Zittel, op. cit. p. 456. 

 1887. Agassiceras, Haug, Polymorphidse ; Neues Jahrbuch fiir Mineralogie, etc. 



Bd. ii. p. 94 [pars). 

 1889. Agassiceras, Hyatt, Genesis Arietidse ; Smithsonian Contrib. Knowledge, 673, 



p. 194 {pars). 

 1893. Paroniceras, Bonarelli, Sul Toarciano, etc. ; Boll. d. Soc. geol. italiana, 

 vol. xii. fasc. 2, p. 202 (10). 

 Also Phylloceras, Blake, and Wright, pars. Earpoceras, pars, Menegbini et 

 auct. Pcecilomorphus, Bonarelli, Pelecoceras, Haug, etc. 



1 See the author's previous paper, Geol. Mag. July, No. 361, p. 298. 



