BASAL PLATES IN CRINOIDEA CAMERATA 499 



the pronounced enlargement noted in the radial cycle, its diameter 

 being neither increased by growth of its component parts nor 

 augmented by their separation from one another; but, as the 

 ventral disk expands, the orals become separated from the radials 

 upon which they were previously superimposed and are carried 

 upward and relatively inward and the costals and lower distichals 

 are incorporated into the cup. The space now existing between 

 the radials and orals generally remains as a simple, membranous 

 perisome, traversed by the five ambulacral canals; but, in some 

 specimens of Antedon rosaceus, 1 and in other modern crinoids, 

 well-defined groups of interradial plates develop in the angles 

 between the brachials. When these plates appear in the cup they 

 are known as interbrachials, and in the tegmen as interambulacrals. 

 Further tracing of the development of the basals and the orals 

 and consideration of stem formation are not here necessary, 

 although they may at times be referred to in the following dis- 

 cussion. 



In tracing the phylogeny of the Batocrinidae and Actinocri- 

 nidae, the anal plate is found as a constant characteristic and 

 the base throughout the series is hexagonal. Complete incorpora- 

 tion of the ambulacral grooves has taken place in the tegmen, and 

 the arms are incorporated in the cup up to and often beyond the 

 second distichals. In Tanocrinus, a genus probably closely related 

 to the ancestors of the Batocrinidae, 2 five basals are present; the 

 anal plate separates the posterior radials, and is in apposition with 

 and trunkates the posterior basal. 



In Xenocrinus, Comprocrinus, and Abacocrinus only four basals 

 are present, the anterior pair being obviously united and somewhat 

 reduced in width. In the other genera of the Batocrinidae and 

 Actinocrinidae there are three equal basals, the basal sutures meet- 

 ing the antero-lateral radials and the anal plate. Why the third 

 basal suture meets the anal plate will be considered later (Plate III). 



The Melocrinidae, while not showing as complex a basal evolu- 

 tion as that shown in the hexagonal Camerata, are interesting in 

 showing the absence of an anal plate in contact with and trunkating 

 the posterior basal, and in some genera an absence of all plates of 



1 Ref. 35, p. 540. 2 Ref. 6, p. 164. 



