1885.] NATURAL SCIENCES OF PHILADELPHIA. 233 



makes an exception to the general rule, by either not touching 

 the basals at all, or only toward the right, as in most of the 

 Poteriocrinidae and Cyathocrinidae, while the lower left side abuts 

 against the azygous plate. 1 In still others, one or more of the 

 radials are compound, consisting of two sections, horizontally 

 connected by suture, which, combined, have about the form and 

 size of the adjoining single radials, and are succeeded by the same 

 number of brachials as the others. This peculiar structure, which 

 to some extent disturbs the general symmetry, and which occurs 

 throughout different families, but onty among Silurian and Lower 

 Devonian genera, is evidently of some-importance as representing 

 a very early phase of these Crinoids. The lower segments are 

 probably embiyonal plates, which were resorbed by the upper 

 segments, i. e., the permanent radials ; in a similar manner as the 

 az}'gous and anal plate are resorbed by the right posterior radial, 

 which in most of the earlier Inadunata either is missing, as in the 

 case of Baerocrinus, or, as in others, imperfectly developed. In 

 Baerocrinus 2 one of the earliest known Crinoids, the azygous 

 piece forms a continuous ring with its four radials, and has the 

 same proportion. In the allied Hoplocrinus, however, the right 

 upper corner of the azygous plate is absorbed and replaced by a 

 small trigonal arm-bearing piece, the right posterior radial ; the 

 left corner of the plate remaining intact. This is taken up by the 

 anal piece in Hybocrinus. In Dendrocrinus the azygous plate is 

 reduced to the size of the posterior radial, with which it is con- 

 nected by a horizontal suture. In Homocrinus this suture assumes 

 a sloping position, thereb} 7 again decreasing the proportions of 

 the azygous plate. In Poteriocrinus the latter is reduced to quite 

 a narrow piece, and the radial toward the right is almost as large 

 as that on the opposite side. In Cyathocrinus and Graphiocrinus 

 the azygous plate has disappeared entirely, and both posterior 

 radials are equal in size, but separated b} _ an anal piece. In 



1 The term " azygous plate " is used here, and throughout Part III, exclu- 

 sively for the uusymmetrical lower plate of the posterior (anal or azygous) 

 interradius, the so-calk'd " first anal plate" of most American writers. We 

 reserve the term "anal piece" for the plate enclosed within the ring of 

 radials. 



2 For further information on Baerocrinus and the gradual resorption of 

 the azygous and anal plate in the Inadunata generally, we direct attention 

 to our paper on Hybocrinus, Hoplocrinus and Baerocrinus ; Amer. Journ. 

 Sci., 1883, vol. xxvi, p. 365. 



