332 Evolution and Distribution of Fishes 



central and southern English territory, two adjacent sets 

 of beds are known as the lower or "angulatus" and the 

 upper or "Bucklandi" zones, since in their marine constitut- 

 ents they are characterized by two species of ammonite (//. 

 angulatus and A. bucklandi) . But the constitutent beds of 

 each of these are by no means uniform. In the "Bucklandi" 

 group especially, some are freshwater — not estuarine — 

 some are marine. The relation of these is succinctly given 

 thus by Phillips-Etheridge (2^0:368) "Immediately suc- 

 ceeding the thick "angulatus" limestones, commences the 

 rich series of the Bucklandi or Lima beds, here about 100 ft. 

 thick, and composed of 70 alternating beds of shale and 

 limestone, some of the shaly beds measuring from 2 to 15 

 feet. These Bucklandi shales have yielded nearly all the 

 fish and saurian remains for which this locality is famous. 

 (Ital. present writer). Twenty beds of grey semi-crystal- 

 lized and earthy limestone alternating with dark shales, 

 comprise the whole thickness of the Bucklandi zone here" 

 (i.e. at Lyme Regis). 



The often large slabs of Liassic reptilian remains that 

 adorn the cases of some museums, belong to this one, and 

 the absence of marine invertebrates from these is note- 

 worthy. 



Succeeding to Mesodon Uassicus, two or three species 

 occur in the freshwater beds of the Stonesfield Slates, and 

 still higher in the Corallian beds. But the climax of de- 

 velopment for the genus evidently occurred in the Kim- 

 meridgian period, for six to nine species are listed from 

 an area that would fairly well include a belt across Central 

 Europe. Of these Mesodon macropterus is most perfectly 

 known, for well preserved specimens have been found in 

 the Solenhofen rocks. Fig. ^^ shows a striking restora- 

 tion of it, for the anterior body scales are alone developed. 

 The genus then decreased markedly in the freshwater Port- 

 land and Purbeck beds, being represented in the latter by 

 Mesodon daviesi of the "shell limestone" and "Paludina 

 clay." It then seems to have died out at the close of the 

 latter period. 



The genera Microdon, Mestwus, and Athrodon can 

 be treated together as being closely related genera. Most 



