In Carboniferous and Permian Epochs 155 



distinctness of the species or at times of the genera. So 

 whether we compare lists gathered across the world from 

 the Eastern States to Australia, or from Canada to Cape 

 Colony the general resemblance is striking. Further this 

 fauna was to a large degree freshwater, the elasmobranchs 

 alone being the group that still sent voracious and predatory 

 outliers into the seas of the period. The number and affini- 

 ties of these are set forth in Chapter 9 (pp. 279-80). 



Even the primitive sharks like Cladodiis, Chomatodus, 

 Diplodus, PleiirodiiSj and Ctenoptychius were still lake- 

 dwellers, or were anadromous in some species, or fresh- 

 water in some and anadromous in other species of a genus. 

 The remains of many of these therefore belonged to the 

 groups Ichthyotomi, Petalodontidae, and Psammodontidae, 

 which were loosely cartilaginous, but can be recognized in 

 their hard teeth and plates, which are often intermingled 

 beside freshwater chondrosteans, dipnoans, and crossoptery- 

 gians. 



J. W. Davis gives {iio: S^) a graphic picture, derived 

 from study of the Cannel Coal of Yorkshire. This forma- 

 tion, as he states, was due to decay of abundant vegetation 

 that became "aggregated in a small inland lake, very 

 shallow and liable to be dried up. The plants forming the 

 coal were washed into this lake by streams, and becoming 

 decomposed, and settling to the bottom, accumulated in 

 a homogeneous mass, prior to its being changed by pressure 

 and chemical causes into coal. The interlamination of 

 shales, more frequent and thicker near the sides of the lake, 

 would naturally result from the mud, also brought down by 

 the streams, settling to the bottom more quickly than the 

 leaves of the plants, but at the same time carrying down 

 with it a large percentage of carbonaceous substance. In 

 some parts the lake appears to have become filled up or 

 elevated above the water-level; and seat-earth filled with 

 Stigmaria rootlets was the result. From the seat-earth 

 grew plants whose remains *have formed thin bands of 

 ordinary coal. 



"After the accumulation of the decaying vegetable 

 matter, sometimes deposited in water and forming cannel 



