240 PROCEEDINGS OF THE ACADEMY OF [1879. 



are situated in line with the radials, and hence cannot be the true 

 basals. He holds that the same order of plates cannot be radial 

 in one genus and interradial in another. This argument is un- 

 questionably a very strong one, and we are enabled to confirm it 

 by a number of interesting observations. 



Let us consider the first ring of plates resting upon the upper 

 stem joint in Crinoids, where it consists of less than five, as a 

 metamorphosed representative of a set of five plates, in which two 

 or more have been united by anchylosis. It will be observed 

 that in nearly all types with but one ring of plates below the 

 radials, i.e., forms without "subradials," the proximal ring is so 

 placed that the angles alternate with those of the radials, so that 

 the whole set, whether five plates or not, may very appropriately 

 be considered as interradial with regard to the general structure 

 of the body. 



In forms like Platycrinux, Symbathocrinus, etc., in which the 

 radials rest directly upon a basal disc composed of three unequal 

 plates, if we bisect the two larger, we obtain five equal plates, 

 which occup}^ an interradial position. This is actually done in 

 Belemnocrinus, which in the apical system has the identical struc- 

 ture of Bhizocrinus, and most of the i*ecent Crinoids, viz., five 

 basals, interradially situated, supporting five radials. 



In 3Ielocrinas and Eucalyptocrinits, where the proximal ring 

 consists of four plates, we may divide the larger one and thus 

 obtain five plates, which, though not wholly regular in form, are 

 all interradial in position. 



In Actinocrinus which has three equal plates in the basal disc, 

 and Dichocrinus which has two, and allied genera, an apparent 

 difficulty is presented, for if we subdivide these plates we have 

 six basals instead of five. But here the structure is precisely the 

 same in principle as in the foregoing cases. The six plates are 

 interradial in position, and the presence of the anal plate in the 

 same range with the first radials necessitates an additional plate 

 in the basal ring for its support, so that the two plates which sup- 

 port it are equivalent to one. The anal plates are actually nothing 

 more than an interradial series with a special function, viz., the 

 support of the anal apparatus within the body, and of an opening 

 in the vault, which may be either directly through the test, or 

 prolonged into a tube. In some of the Ichthyocrinidee, the anal 

 is not distinguishable from any interradial series. The presence 



