REPORT OF THE DIRECTOR I915 I37 



In Urasterella the cover pieces could be thrown back far 

 enough to hide the inframarginals and the latter could be crowded 

 over nearer the supramarginals. They were set slightly in advance 

 of the floor plates to which they belonged and each, viewed orally, 

 slightly overlapped the next proximal floor plate. The first cover 

 plates met over the single axillary inframarginal to form an inter- 

 radial jaw. The members of these " mouth angle plates " were not 

 so highly specialized as to hide their origin, but their movement was 

 in arcs of circles lying in vertical axillary planes. They could 

 thus very effectively thrust captured material into the mouth. 



In Protopalaester the opening and closing movements of the 

 cover plates was very limited. They were set alternately with the 

 floor plates but viewed orally, each slightly overlapped the next 

 distal floor plate. This form of imbrication, save for arm tips in 

 a few cases, is unique and requires explanation. The interradial 

 pairs were heavier than the others and also apparently less free in 

 their movement than in Urasterella. 



So far as the peristomial ring and plates of the food groove are 

 concerned, Urasterella is the more simple and primitive of these 

 two forms. The floor plates begin where the groove begins and 

 the cover plates arch over the mouth. The rigid parts of the ring 

 consist of but fifteen pieces, ten cover plates and five axillary 

 inframarginals. This ring is strong but allows a small, radial 

 gliding motion of the floor plates, which enables the first pairs to 

 function as radial jaws. The first cover plates become slightly 

 modified mouth angle plates and function as Interradial jaws. 

 Urasterella is then a very generalized form having both types of 

 oral armature. 



In Protopalaeaster there is no oral peristomial ring. The 

 axillary Interradials are braced by the supramarginals and radials. 

 radlals. The first, second and third floor plates are parted from their 

 opposite members and swung over toward the interradil. The 

 strong elements of the jaws, visible from the oral face, are the first 

 cover plates which no longer rise and arch over the opening. On 

 their apical surfaces rest the paired and weakened first floor plates. 

 Thrust over these again are the second floor plates. Are these 

 first and second floor plates vestigial or do they really serve some 

 Important function? On the supposition that he was viewing the 

 oral face of P. narrawayl, the author recognized the remark- 

 able similarity in arrangement between what he took to be epln- 

 eurals In this genus and the undoubted cover plates of Urasterella. 

 It was an easy matter also to conceive of a function for these 



