392 PROCEEDINGS OF THE ACADEMY OP [1881. 



The arm openings are situated in rather large, deep depressions, 

 lying under the edges of the pseudo-brachial appendages, but only 

 when the latter are very large, on either side of them when they 

 are small. In the latter case, so far as observed, the arms are 

 erect and folded over the dome, in the former pendant, but in 

 either case the ambulacral furrow lies to the inner or under side 

 of the arms, close to the body, showing the pendant position to 

 be the normal one. 



Vault depressed, generally flat, composed of numerous, some- 

 what convex, polygonal plates, among which the apical and 

 radial dome plates may be readily distinguished by their greater 

 size. The interradial regions are occupied by a large number of 

 small pieces, which are generally found more or less depressed 

 just in front of the base of each false arm. The structure is 

 such that these parts may have been flexible and capable of con- 

 traction or expansion. Anal opening directly through the vault, 

 not protruding, situated about midway between the centre and 

 the margin. 



The margin of the dome above, and generally between the arm 

 bases of different rays, is extended into five pseudo-brachial 

 appendages, which near the body are comparatively heavy, 

 nearl}^ solid, transversely elliptic, composed of two rows of 

 semi-elliptical joints, rounded on the outside, and joining in the 

 middle by straight, vertical faces. At from four to eight joints 

 from the body, each trunk divides into two rounded branches 

 tapering to a point, which are perforated throughout their length 

 by a small central passage, these passages enter the body in 

 pairs through the main appendages, and thence diverge toward 

 the arm furrows, with which they unite. There are no other 

 openings, nor any external furrows, these organs being simply 

 heavy calcareous tubes, with a jointed structure like that of the 

 column. 



Column round, composed of joints alternately larger and 

 smaller ; central perforation of medium size, pentagonal. 



Geological Position^ etc. — Ollacrinus is essentially a Subcarbon- 

 iferous genus, all the known species being from rocks of that age, 

 except a single species which is from the Hamilton group of New 

 York, and this is a transition form. It occurs both in Europe 

 and America. 



We recognize the following species : — 



