460 



DR F. A. BATHER. 



§ 389. Distally the stem is often coiled in the plane of extension (Brit. Mus. 

 E7683, P. elegans). Whether it tapered ofif or not is unknown ; no specimen has yet 

 shown any trace of a root. 



§ 390. The Species and their Distribution. — The following table gives the names, 

 with the geological and geographical distribution, of all species hitherto described : — 





Kentucky. 



Ontario. 



Anticosti. 



Tyrone. 



"Wales. 



GiRVAN. 



■a] 2 



Si 







anficostensis 



anglica 



Rugeri 



'procera 

 quadrata 

 gibba 

 foriolus 







squamosa 

 robusta 











si 





filitexta 











11 



mercerensis 



elegans 











5 £ 



3 H 





exornata 







^ 





^"" 





(and 

 ? Chazyan) 











From this it appears that the known distribution is limited in time to the upper 

 three-quarters of the Ordovician ; and in space from the " Ontario basin," down to the 

 " Kentucky Strait," and along the " St Lawrence trough," across to the British Isles 

 (for geographical terms, see Sghuchert, 1910). 



§ 391. Here, then, is a highly specialised genus migrating slowly, so far as we 

 know, from west to east, and becoming still more modified as it travels. It is not 

 surprising that it should have given rise to a number of slightly different forms, and 

 that it should be difficult for us to arrange these in clearly defined species. This 

 difiiculty was recognised by E. Billings (1858, p. 48), and has been felt by subsequent 

 writers for the Ontario species : it is no less as regards those from Britain. 



§ 392. Hitherto only two species have been described from this side of the Atlantic : 

 P. Rugeri Salter (1866), from South Wales, and P. anglica Jaekel (1899), which 

 now proves to have come from Ireland. The occurrence of two undetermined species 

 in the Drummuck Group at Thraive was noted by Mrs Gray in Peach & Horne 

 (1899, p. 687). Her collection now contains some sixteen specimens, which I refer to 

 four new species. 



§ 393. This makes six the total for Britain, just as it is for America. These numbers, 

 considering the limited distribution, are large, and it may be that further material 

 may enable us to merge some species with safety. For the present I see no middle 

 course between, on the one hand referring all the American forms to two species and 

 all the British to one (or possibly to two) other species, and on the other hand 

 splitting them as I have done. 



Further discussion of the species, especially of the relations between the British 



