REVISION OF PALEOZOIC STELLEROIDEA. 37 



Spinosity can also be studied ontogenetically because the youngest 

 ossicles do not have spines, and when these appear they are smaller 

 and less characterized than the older ones situated in the proximal 

 portions of the columns. 



Many of the ontogenetic changes observed by the writer are re- 

 corded throughout this work, and their significance in related species 

 and genera has been made use of in discerning the phylogenetic 

 lines of descent. 



Regeneration. Among living starfishes it is common to regrow 

 arms that have been lost through accidental causes. From the 

 base of the severed ray a new growing tip is established, forming 

 a juvenile arm that gradually grows to full size and assumes 

 mature characteristics. Schondorf (1909a: 96-97) states that this 

 habit has been pronounced since the Jurassic but that he has failed 

 to find marked regeneration in Paleozoic asterids. He did, how- 

 ever, note partial replacement of minor losses among the Devonic 

 species. Stiirtz, who has handled more Paleozoic asterids than any 

 other paleontologist, also has not noted a single case of marked 

 regeneration. The same is true for the 400 Devonaster eucJiaris 

 found in a limited area of the Middle Devonic of New York. 

 Clarke, in describing this find (1912: 44-45), however, does note a 

 few specimens " which show the existence of only four instead of 

 the normal five arms." These are the only examples of four-rayed 

 Paleozoic starfishes so far recorded. 



The writer has also been unable to find a single case of regenera- 

 tion, but in the Middle Ordovicic cryptozonian Urasterella ulrichi 

 he describes a specimen with two normally developed rays and three 

 short stumps. All of the arms are normal for the species, except 

 for the length of three rays and their terminations, which are blunt. 

 (See pi. 29, fig. 1.) It seems to him that this occurrence is not due 

 to the accident of fossilization or weathering, but is an actual case 

 of loss in life with subsequent healing of the wounds, but without 

 regeneration of the lost parts. 



During most of the Paleozoic, the starfishes could have had no 

 carnivorous enemies other than the cephalopods; as for marine 

 fishes, the armored Arthrodires did not appear until the Middle 

 Devonic, while the ancient sharks were not common until Lower 

 Carboniferous (Mississippi) time. It is possible that regeneration 

 among the starfishes is connected with the rise of carnivorous ene- 

 mies, but as the habit is so common among living forms it is more 

 probable that this power has always been inherent in the class. 

 Regeneration among the crinids has been noted in several cases 

 where lost distal ends of arms were being replaced by immature 

 growths. Such have been seen in the Lower Carboniferous (Bur- 

 lington and Keokuk formations) of America. 



