340 PROCEEDINGS OF THE ACADEMY OF [1888. 



specimen alone. The real structure of the vault of Haplocrinus is 

 as follows : The five triangular plates composing the ventral pyramid 

 meet in the center by sutures which are often difficult to see. The 

 posterior plate is the largest, and jirojects in between the two postero- 

 lateral ones, completely separating them, and interlocking wdth the 

 antero-lateral plates by a variety of plans, from a simple zigzag suture 

 to a triangular or dovetailed insertion, or a long slender tongue ex- 

 tending into the latter plates, which are cut away to fit it (PI. XVIII, 

 figs. 6^ and 61). This projection stands sometimes at a lower level 

 than the other part of the plate and the adjoining plates, leaving a 

 depression in the center which is sometimes partially occupied by a 

 small node. In other cases a high ridge runs from the posterior 

 plate over the central space, branching to the two antero-lateral 

 plates (PI. XVIII, fig. 61). It thus appears that the whole ventral 

 surface in Haj)locrinu8 is covered by five large plates which meet in 

 the center as in Allagecrinus. 



The anal opening in Haplocrinus has not heretofore been correct- 

 ly identified, but it lias been generally claimed to be located at the 

 suture between two radials and the posterior vault plate. In the 

 Revision III, pp. 157 and 162, we alluded to a small pore we had 

 observed in one specimen of H. mesplliformis, the position of which 

 is indicated in fig. 1, on PI. V, of that work. We afterward became 

 satisfied that this pore was due to chemical action, or some im- 

 perfection in the test and was not organic, as subsequent examina- 

 tion of a very large number of specimens of the same species, better 

 preserved, failed to disclose any opening in that position. After we 

 discovered that the so-called central plate was nothing but a pro- 

 longation of the posterior vault plate, it became easy to distinguish 

 that plate in the specimens, and we began a careful search upon that 

 side, from the radials up, for the anal opening. We soon found a 

 small, scar-like opening or pit, with a slightly thickened and well 

 defined rim, situated just within the upper angle of the triangular 

 depression on the posterior plate (PI. XVIII, fig. 61). A similar 

 structure was observed in a large number of specimens, varying in 

 form from that above described to a small tubercle in which no 

 opening could be detected. It was always in the same position, and 

 Ave have been unable, after the closest examination, to discover any- 

 thing like it upon either of the other four plates in any of the 

 specimens. We gi'ound down a number of specimens on the pos- 

 terior side, and in every one found that this was an actual opening, 



