288 PROCEEDINGS OF THE ACADEMY OF [1879. 



Looking first at the subgenus Iocrinus, one of the earliest 

 known forms of Crinoids, we find the body up to the top of the 

 radials perfectly equilateral, all basal and radial plates having the 

 same form ; but higher up this symmetry becomes disturbed by 

 irregularities in the disposition of the brachial plates. There are 

 four brachials in four of the rays, short and all quadrangular, 

 while the right posterior radial supports a peculiar bifurcating 

 plate of the same width as the other brachials and apparently 

 similarly articulated. Its right sloping side supports four bra- 

 chials, its left a number of large, heavy, almost quadrangular 

 plates, longitudinally arranged, rounded on the back, which have 

 the general appearance of arm plates, and such they have been 

 taken to be by most authors. They are, however, plates of a 

 rather strong ventral sac, and extend to its full length forming a 

 highly elevated ridge (PI. 16, Fig. 3). The plates are bordered on 

 each side bj' about double their number of rather delicate pieces, 

 altogether different and transversely arranged, which from their 

 peculiar elongate form and relative position resemble pinnulse be- 

 longing to an appendage that looks like an arm. Now the ques- 

 tion arises, what shall we call the bifurcating plate which gives 

 rise both to the right posterior ray and to the ventral tube? It 

 was certainly a movable plate, and as we mentioned before, its 

 mode of articulation was evidently the same as that of the bra- 

 chials. Shall it likewise be called a brachial? And again, was not 

 the ventral tube here, and in Crinoids generally, originally a 

 modified arm? This, if true, would at once explain why the anal 

 area leans always toward the right and never to the left side of 

 the bod}'. Is the plate, on the contrary, an anal plate? If so, the 

 arms in that ray would rest upon the base of the proboscis, which 

 is not very probable. That it is a brachial with interradial func- 

 tions, is illustrated in other genera. In D ndrocrinus, as may 

 be seen, PI. 16, Fig. 5, the case is substantially the same, but in 

 that genus a regular anal plate has already appeared within the 

 calyx, supporting the ventral sac. Here four of the radials are 

 equal instead of four brachials, and the fifth is a compound plate 

 consisting of two successive plates of about the form of the simple 

 ones, but slightly larger. Nobodj T can doubt that here both the 

 upper and lower sections of the compound plate, which are sepa- 

 rated by a horizontal suture, are strictly radial. Looking, however, 

 at Homocrinus, PL 16, Fig. 6, it will be found, although it diners 



