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



(K) General Results. 



The distribution of the ammonites in the Shales-with-Beef is 

 summarized in the table facing this page. It will be seen that the 

 family Arietidse is dominant throughout, as it is for the whole of 

 the Lower Sinemurian. Of the five genera of this family, two 

 range up from beds below the limits of the section here described : 

 namely, Agassiceras and Arnioceras (sensu lato). The true 

 Arnioceras (including the well-known A. geometricum), in fact, 

 characterizes beds in the bticMandi zone very considerably below 

 the Agassiceras beds; on the other hand, the very late A. (' Epar- 

 nioceras' 1 ) semicostatum, which in museum specimens occurs 

 associated with Xipheroceras, has not yet been found in place, and 

 may prove to belong to beds even higher than the highest Arnio- 

 ceras bed of the table. Arietites and Cgmbites range up into beds 

 above the limits of the section here described, but probably not very 

 high, for Asteroceras soon becomes the dominant Arietid genus. 

 The only Arietid genus, then, proper to the Shales-with-Beef is 

 Par 'arnioceras, here considered to be a specialized offshoot of 

 Agassiceras. It is also the only genus in these shales that 

 commonly reaches large dimensions, although one fragment of an 

 Arietites broolci of gigantic size has been found by Dr. Lang in 

 the brooki bed, and large Microderoceras, of course, occur again 

 in the foVc/w-nodular, near the top of the section. 



The Schlotheimid genus Sulciferitcs and the two Deroceratid 

 genera Microderoceras and Xipheroceras must be looked upon as 

 crvptogenous genera (in Neumayr's and Haug's sense), which have 

 probably immigrated from the Mediterranean Province. The 

 enormous quantities in which some of these shells, as, for example, 

 Microderoceras birchi, are found in certain beds, including all 

 stages of growth, indicate that the organisms lived on the spots 

 where their shells now occur, and that there was no ' transgressive 

 distribution ' of drifted shells from other regions, as Prof. J. 

 Walther held. 1 



As regards the two Deroceratids, it has already been mentioned 

 that the Mediterranean genera Ectocentrites and Derolytoceras (?), 

 with which the family Deroceratidge is here connected, occur at 

 Spezia and the Monte di Cetona in beds juelding abundant Arnio- 

 ceras and Sulcifh'ites. It may be assumed that these beds are 

 homotaxial, if not actually isochronous, with the Dorset strata. 

 Other new types found in Italy, however, as, for instance, Hypas- 

 terocehas, gen. nov. (genotype : Asteroceras (?) ceratitictim 

 Fucini, pt. iii, Pal. Ital. 1903, pi. xxiii, figs. 1 a-1 c) or the forms 

 probably misidentified as ' VermictrasJ make it appear probable 

 that the succession between the Agassi ceras-saazeamini beds 

 below and the Xipheroceras-planicosta beds above is more com- 

 plete then than it is at Charmouth. For the present, however, it 



1 ' Einleitung in die Geologie, &c.' pt. ii (1893) p. 516. 



