178 Evolution and Distribution of Fishes 



of the Lower Rhaetic were deposited as alternating fresh- 

 water and marine deposits with typical and quite distinct 

 fossils for each. Then follow some alternating layers of 

 black, shale and of bone beds rich in fish remains but all de- 

 void of marine organisms. Along with the fish, or in adja- 

 cent beds, abundant remains of Estheria miniita and a plant 

 Lycopodites are recorded. The fish remains include Acro- 

 dus minimus, Hybodus minor, H. cloacinus, Gyrolepis al- 

 berti, Saurichthys alberti, and Sargodon tomicus, the laby- 

 rinthodont Mastodonsaiiriis — that at once indicates fresh- 

 water conditions, — small teeth of Sphaerodus and "a man- 

 dible believed to belong to Palaeosaiiriis, as well as other 

 saurian remains." 



The beds of the Upper Rhaetic seem to suggest a 

 marine invasion, but unless the above-named author has 

 mixed up contents of freshwater beds with these, the 

 presence with Pecten, Avicula, Pteria and Aviciiloidea, of 

 Gyrolepis scales suggests either that some hitherto fresh- 

 water species were slowly invading seashores, or as is much 

 more likely that the last were washed out from older strata 

 and redeposited in marine beds. 



So apart from fishes, large amphibians and reptiles 

 allied to those of the Permian, like Mastodonsaiiriis, Tre- 

 matosaurus and Hyperodapedon, waded in the swamps, or 

 basked in the freshwaters, or progressed over the muds 

 deposited from recent freshets of the lakes or rivers. Thus 

 they left "footprints on the sands of time" that to-day 

 have enabled palaeontologists to distinguish many species, 

 even in absence of the animal remains. Such equally applies 

 to the reptiles, that now are represented by the three great 

 divisions of the crocodiles, the lizards, and the turtles. 



First evidence of mammals is seen in the small form 

 Mtcrolestes, which once was regarded as a primitive mar- 

 supial, but as only fragmentary remains of the skull are 

 known, it may equally well have been a primitive mono- 

 treme. 



In passing now to other regions where the Triassic was 

 represented by freshwater beds, it may first be well to in- 

 quire as to the possible extent geographically of the system. 



