1912] The Ottawa Naturalist. 51 



but it was curved toward the aboral surface. The joints formed 

 by contact of the first arm marginals with the interradial mar- 

 ginals are all gently concave aborad and suggest sliding or 

 shallow ball and socket joints. The movement may have been 

 something like that of the Ophiuroidea, the side arms being 

 lifted and set forward and the epineurals holding like anchors 

 or helping in the forward thrust. In that case our orientation, 

 based in part on arm position, may be in fault. Aside from the 

 ability of the long epineurals to open widely and close, the angle 

 at the fixed end of about 85 degrees indicates an ability to swing 

 their free ends through an arc of some 95 degrees in a radial 

 direction. We have already noted that the preserved plates on 

 the two arms are set in opposite directions if considered 

 radially. With reference to the environment, however, they are 

 set in the same direction and are in the position we should 

 expect if they had been used to assist in thrusting the creature 

 in the direction of the third interradius. 



Some General Considerations. 



We all know that the more primitive Echinodermata pos- 

 sessed food grooves with covering plates such as we find in Cys- 

 tidea, Crinoidea and Edrioasteroidea. In 1907 1 I described the 

 covering plates in Parablastoidea (Blastoidocrinus) and in 191 1 2 

 after further work on the same species I endeavored to show 

 that with regard to Pentremites we must "accept Doctor Car- 

 penter's contention that the mouth, food grooves and pores were 

 covered with small but well fitting plates." We now have found 

 undoubted covering plates in the Stelleroidea. 



I desire to point out that the food groove with a double 

 row 7 of flooring plates covered by a double row of epineurals and 

 flanked by one or more marginal plates on either side is a very 

 primitive type of food groove and I believe that Protopalaeaster 

 narrawayi not only points out the fact that the Stelleroidea 

 arose from such a type but that the Echinoidea also had a similar 

 parentage. 



With so simple a form before us we must ask ourselves if 

 the ambulacra and interambulacra of Echinoidea and the "verte- 

 bral ossicles" and "lateral arm plates" of Ophiuroidea are not 

 strictly homologous with the adambulacra and marginals of our 

 type and of the Edrioasteroidea. 



The very evident specialization of the peristomial covering 

 plates of P. narrawayi for food capture and mastication would 



1 " On Some Pelmatozoa from the Chazy Limestone of New York." 



In New York State Museum Bulletin 107, p. 112. 



2 " Studies of some Early Seluric Pelmatozoa." 



In New York State Museum Bulletin 149, p. 208. 



