104 PROCEEDINGS OF THE ACADEMY OF [1886. 



of a single row of from 2 to 20 heavy, transverse pieces, of which 

 the upper and lower sides are parallel. The arm plates grow 

 narrower and shorter at their upper ends, they are suturally con- 

 nected, and sometimes so closely anchylosed that no suture lines 

 can be distinguished along the lateral margins of the arms. 



The radials are connected with large muscle-plates (PL 4, fig. 

 1), which are laterally anchylosed, and together form one con- 

 tinuous plate. This covers the greater part of the ventral 

 surface, leaving in the centre but a comparatively small, irregu- 

 larly pentagonal open space, which, as we suppose from analogy 

 with allied forms, was covered in the animal by interradial and 

 summit plates. The median portions of each radial is pierced by 

 two successive, rather large openings, of which the outer ones 

 penetrate the upper margin of the plate, and serve as passages 

 for the axial cords ; the inner ones, which pass through the 

 muscle-plate, probably are ambulacral openings. The muscle 

 plate in its outer form is stellate, having five pairs of leaf-like 

 extensions, two to each radial. They contain upon their outer 

 faces indistinct striae, which from both sides concentrate toward 

 the axial opening. The leaves become more delicate toward 

 the periphery, especially interradially, where they are frequently 

 broken, and the spaces, thus formed, appear in the specimens as 

 if constituting natural openings. 



The articulation of the arms with the calyx was facilitated 

 b}^ the axillaria, which, like the radials, have an axial canal, 

 and, according to Schultze, similar muscular processes. The 

 articular face of the arms is truncate, sloping toward the inner 

 side, and corrugated for the reception of ligament. The arm 

 furrow is deep and wide, and contains along the inner floor an 

 unusually large axial canal, which, to the extent of the proximal 

 arm piece, is partitioned off from the ambulacral groove. The 

 exact construction of the ambulacra is not known, but we judge 

 they were plated in a similar manner as those of Symhathoc7Hnus, 

 and not membranous as described by Schultze. They probably 

 also were provided with lateral pores, and these were alternately 

 arranged with incurved, articulated pinnules, resembling in form 

 those of the recent genus Holopus, but arranged like those of the 

 Blastoids, there being four or more to each arm joint. 



Gupressocrinus has been described to possess no anal plate, 

 only an anal opening, located between two of the muscle plates. 



