62 CANADIAN FOSSILS. 



two or three rows of these punctures upon each suture. The greater 

 portion of the area of the plate is marked with deep fissure-like striae 

 at right angles to the suture, and with thin erect lamellce or partitions 

 between them. These are sometimes crossed by other lamellae 

 parallel with the edges of the plates, the effect of which is to 

 produce a peculiarly rough surface. Sometimes none of these are 

 visible, and the surfaces of the plates are then uniformly smooth and 

 solid. These variations are the results both of weathering and 

 structure. Portions of the centres of the plates of the two specimens 

 figured (pi. V. figs. 1 and 2) have the external surface smooth, while 

 the margins are rough. 



The mouth is large, near the apex, and closed by a pyramid of 

 five triangular valves. 



The ambulacral orifice has not yet been distinctly observed, but 

 there can be little doubt of its existence. The arms are four in 

 number, and consist of an anterior pair situated directly over the 

 mouth, and a posterior pair placed opposite, on the posterior side of 

 the summit ; a deep narrow groove crosses the apex, in a direction 

 from the anterior to the posterior side ; from one end it sends up 

 two branches into the anterior pair of arms, and from the other end 

 two into the posterior pair. The arrangement of the ambulacral 

 furrows in this species is then precisely as in Glyptocijstites midtiporiis, 

 and no doubt an orifice will yet be found in the bottom of the 

 groove, upon the summit between the two pairs of arms. The arms 

 consist of a single series of joints, each about one line and a-half in 

 length; the pinnulae are nearly cylindrical, and divided by joints at 

 lengths of about half a line. In the only specimen known with the 

 arms attached (plate v. fig. 1) one side of the arm only can be seen, 

 the other being imbedded in the rock. It shews that upon at least 

 the side exposed there is one pinnula to each joint, but whether 

 there is a row on the other side of the groove or not remains to be 

 ascertained. 



The column is round and smooth, formed of very thin joints, and 

 does not, in a specimen with three inches preseiTed, exhibit any 

 signs of tapering. 



Explanation op Figures. Plate V. 



Fig. 1. Anterior view of a large specimen, with one arm and the pinnulfe attached. 



0, the mouth ; la, a plate of the same enlarged, to shew the peculiar stria- 



tion ; lb, the valvular apparatus of the mouth enlarged. 

 Fig- 2. View of the left side of another specimen; 2a, a plate of same enlarged; 



26, outline of the summit, shewing the positions of the bases of the arms, 



and the ambulacral groove between them. 



