E. H. L. Schwarz — Shell- Structure of Ammonites. 249 



they are most closely related to the Dufton Shales of the Cross Fell 

 area, and to the Sleddale Beds of the Lake District proper. 



Lithologically they do not differ more among themselves than do 

 the Dufton Shales, which they resemble closely, since both are in 

 the main composed of dark, flaggy, calcareous mudstones. The 

 preceding table shows the species common to the Drygill Shales, 

 Dufton Shales, the Sleddale Group, etc. 



Summary. — The result then of our examination of the fossils of 

 the Drygill Shales is to show the predominance of Bala Limestone 

 forms, and the close parallelism of the beds with others of the same 

 age in the Lake District. 



Although correlated with the Bala Limestone as a whole, we 

 think that the Drygill Shales are capable of further subdivision, 

 viz. into (1) an Upper Group, characterized by the abundance of 

 TrinucJeus ; (2) a Lower Group, characterized by the abundance of 

 Ampyx and Orthis testudinaria, though it is not suggested that this 

 subdivision is of more than local importance. 



Note. — We take this opportunity of thanking Mr. Marr for many 

 valuable suggestions, and of gratefully acknowledging the help given 

 us by our friend Miss E. G. Skeat of Newnham College, in the 

 identification of some of the fossils. 



Our list of fossils was compiled from collections made at various 

 times from Drygill, all of which are now in the Woodwardiaa 

 Museum. 



IV. — Shell-Struottjre in the Ammonoidea. 



By Ernest H. L. Schwakz, A.R.C.S. 

 From the Geological Research Laboratory, Royal College of Science. 



THE shell-structure of the Ammonoidea has received very in- 

 adequate attention from palaeontologists ; in most cases authors 

 are content with describing the shell of the recent Nautilus, and 

 saying that that of the Ammonites is exactly the same. Prof. Judd 

 therefore suggested that I should take the subject up, when working 

 as a research student under him. As a result, I have found that the 

 resemblance of the Nautilus- to the Ammonite-shell is literally true; 

 but the structure of the extinct forms throws an altogether new light 

 on that of the recent one, while the usual homology of parts between 

 the two requires, I think, revision. 



In the Nautilus there are two main layers composing the walls of 

 the shell : one nacreous and inner, the other porcellanous and outer. 

 Of these, that corresponding to the inner layer is usually preserved 

 in the Ammonoidea. In the Clymeniidse, no details of shell-structure 

 have been published; but in the Goniatites, Hyatt ^ has figured 

 a Gon. (Ghjphroceras) crenistria, Phill., from Kudesheim, showing 

 the two layers, but both are probably parts of the same layer, since 

 mineralization has destroyed the original structure. 



In the Ammonites proper, the earliest example showing the 

 original lustre and structure on record is Ceratites nodosus, de Haan, 



1 Biill. Mus. Comp. Zool. Cambridge, iii, 1852. 



