URANASTERID^E. Ill 



probability Lower Arenig. This is based on the identification in them of Calymene 

 tristani and Ogygia selwyni, both characteristic Lower Arenig forms. In their 

 rather distorted state Hicks believed them to be Neseuretus and Niobe respectively. 

 . . . There are probably no Tremadoc beds anywhere in the St. David's district, 

 the Arenig beds being either faulted against Lingula Flags or older rocks, or lying 

 unconformably upon them." This determines the horizon of the species to be 

 Lower Ordovician, but even with the newer age of the form established, U. ram- 

 seyensis is the oldest known Asterozoan. 



American Forms of Uranasteridse. 

 It has already been pointed out (pp. 105, 106) that the American form 

 Petraster speciosus is very similar to the English species of Uranaster. Schuchert 

 has given a very full description of the form (85, pp. 142 — 145), to which the 

 reader is referred. One of his illustrations is reproduced here (Text-fig. 67). 





Text-fig. 67. — A portion of the apical surface of Peirasler speciosus to show the stellate supero-marginalia 

 just inside the bordering infero-marginalia (reproduced from Schuchert). x 2 - 5. 



It is clear from Schuchert's description that P. speciosus is descended from a 

 slender-rayed form very similar to a Uranaster. It might be argued that since 

 P. speciosus is from the Upper Ordovician and that U. ramseyensis and U. linahani 

 are from the Lower and Middle Ordovician respectively, P. speciosus might be 

 a descendant of the English forms of Uranaster. Personally I am not inclined to 

 take this view, for the appearance of the distal region of the arm is dissimilar in 

 P. speciosus from that in the two species of Uranaster. None of the characteristic 

 half-moon-shaped plates are present in the American species. Further, the 

 mouth-angle plates in Uranaster are relatively larger than in P. speciosus. Rather 

 does it seem that Uranaster and P. speciosus have descended from a common stock 

 which has undergone slightly different changes in the two regions. 



The type species of Petraster is P. rugosus, Billings, but unfortunately it is so 

 little known that we cannot argue as to its exact relationships. I originally 

 intended to make generic comparisons between Uranaster and Petraster, but until 

 more is known of the type species of Petraster this appears to be useless. The 

 species described by Schuchert as Petraster ? americanus (D'Orbigny) does not 

 appear to be related to either P. sjieciosus or P. rugosus (see p. 125). 



