38 



THE PSILOPHYTON FLORA 



[CH. 



Distribution. Upper Devonian of Belgium, United States, 

 Canada and Australia. 



This fertile shoot is at present 

 wholly obscure, but it is very un- 

 like any other organ known from 

 more recent rocks. It is widely 

 distributed in Devonian rocks. 



Parka. 

 (Fig. 20.) 

 Parka, Fleming i, 1831. Body 

 small, of variable size, rarely ex- 

 ceeding one or two inches across, 

 more or less circular, lenticular, 

 of very small thickness, containing 

 many disc-like oval or circular 

 masses, which in their turn contain 

 spores. 



Distribution. Silurian and Lower 

 Old Red, Scotland and England. 

 Not known outside Britain. 



Don and Hickling^, in an im- 

 portant and quite recent paper on 

 this mysterious fossil, have shown 

 conclusively that the disc-like masses of Parka undoubtedly 

 contain spores and that the fossil is thus clearly of vegetable 

 origin. In this matter they confirm the conclusions of Dawson 

 and of Penhallow^ and more recently of Reid, Graham and 

 Macnair^. Don and Hickling show that Parka consists of a flat, 

 dorsiventral, multi-cellular and multi-layered thallus, develop- 

 ing by marginal growth (Fig. 20, 1). The shape is roughly 

 circular or oval ; but lobate, reniform and even irregular forms 

 occur. In size the thalli measure 5 mm. to 7 cm. across. The 

 margin, Avhich is distinctly frilled, is usually less than 1 mm. 

 broad. The rest of the thallus is composed of small oval 



1 Fleming (1831). " Don and Hickling (1917). 



3 Dawson and Penhallow (1891); Penliallow (1892). 

 * Reid, etc. (1897) and (1899). 



Fig. 19. Bdriiiojiii/jton Bichard- 

 soni, (Daws.), from the Upper 

 Devonian of the United States. 

 After White (1905). 



