188 PROCEEDINGS OF THE ACADEMY OP [1881. 



to avoid confusion, to nse the term " interaxillary " for these 

 plates. 



In the Platyeriniclffi, which have rare]}' more than a single order 

 of radials within the calyx, interaxillary plates are not represented. 

 In the ActinocrinidjB and Rhodocrinidfe the}^ may be present or 

 absent in the same species, and sometimes in different rays of the 

 same specimen, their number, like that of the interradials, increas- 

 ing with age. 



Roemer and Joh. Miiller considered the presence of interaxll- 

 lai'ies as of generic value, and the latter proposed a division of 

 the genus Actinoci'iniis, placing all species having those plates 

 under Pyxidocrinus; but it is evident that such a division is alto- 

 gether artificial and not warranted by the facts, and if carried out 

 would produce confusion. 



5. Yault. 



One of the writers, in a paper upon " the internal and external 

 structure of Paleozoic Crinoids " (Am. Jour. Sci., Sept. 187*7), 

 discussed the importance of the vault with reference to classifica- 

 tion. It was noted that in a large number of genera, among them 

 Actinocrimis, Rhodocrinus and Platycrinus, and their allies, the 

 ventral coA'cring is composed of strong plates closely cemented 

 together, and that these form a free arch which braces the entire 

 oral side of the body without the aid of oral plates. This is the 

 general character of the vault in the family under consideration. 



The vault in the Sphseroidocrinidae is usually well preserved, 

 owing to its solid structure, and is capable of accurate definition. 

 Its plates var}' from a few to many hundred ; but, notwith- 

 standing this diversity- in number, their arrangement is governed 

 by definite rules. Certain of these plates, which we have termed 

 the " apical dome plates," are represented in every species of this 

 group. They consist of a central piece, occupying a position 

 directl}'' above the oral centre, which in this family is quite 

 uniformly the centre of the disk. It is surrounded by six prox- 

 imal plates, interradial in position, of which four are large, and 

 equal, and two smaller. The four large plates are placed above 

 the four regular interradial spaces respectivel}^ ; the two smaller 

 plates which are equivalent to and take the place of one large plate, 

 are directed posteriori}-, being separated from each other by anal 

 plates or the proboscis. These seven plates are easily recognized 

 in species with comparatively few summit pieces and a lateral 



