PALEONTOLOGIC CONTRIBUTIONS 20, 



teatures of both groups, with, a prevailing leaning toward P . 

 squamosus. This, combined with its earlier appearance in the 

 lowest Trenton beds, indicates that it stands nearer the common 

 ancestor than either P. filitextus or P. squamosus. 



It forms a most interesting counterpart of P. filitextus 

 mut. exornata which has all the features of a filitextus, 

 but the pectinated rhombs short. Thus these two earlier mutations 

 bridge, to a considerable extent, the gap between the two Trenton 

 species of Pleurocystites. 1 



The variety anticostiensis is described by Jaekel as alike 

 to P. squamosus, excepting the long pectinated rhombs, and 

 would therefore be identical with m a t u t i n a . It is, however, 

 not from the Trenton as assumed by Jaekel, but from the late 

 Ordovician or early Silurian of Anticosti, and judging from 

 Billings's description and figure its pectinated rhombs are short 

 as in P. squamosus. 2 It. is therefore probably a later 

 derivative of P . squamosus. 



Clarkeaster gen. nov. 



Professor Schuchert 3 has referred a Devonian starfish, described 

 by Clarke and Swartz as Palaeaster clarki, to Mesopalae- 

 aster with doubt because the disk skeleton was not preserved, and 

 stated that it is very probable that when this feature is known the 

 form will be seen to belong to a new genus. We have before us, 

 besides the species mentioned, another closely related form, C . 

 p e r s p i n o s u s , which shows the characters of these species to 

 be such as to require their recognition under a new generic term. 

 Only one abactinal disk is sufficiently well preserved to allow a fair 

 analysis of the plates. This shows a rather large central plate which 

 is surrounded by a ring of ten (eight of which are counted and two 

 indicated by the interspaces) small plates ; this in turn is surrounded 

 by one of ten larger plates of fairly equal size with adjoining, very 

 small, accessory plates. Outside of this follows the ring of five 



1 Dr Percy E. Raymond, in a letter of October 13, 1915, has kindly 

 informed the writer that P. squamosus and P. filitextus 

 occur in what he has called the cystid bed of the Prasopora zone, that is, 

 in the true Trenton, while the mut. exornata comes from the Glens 

 Falls limestone at Montreal, is hence of the same age as matutina and 

 both these mutations are distinctly older than the typical P. filitextus 

 and P. squamosus. 



2 Billings states that the outline of the rhombs is as in P. robustus. 



3 Schuchert, Charles. -Revision of Paleozoic Stelleroidea, with special refer- 

 ence to North America, Smithsonian Institution, Bui. 88, p. 98, 1915. 



