56 CANADIAN FOSSILS. 



There is very little difference in the form of the rhombs. They 

 have each, except the three smallest, a smooth space in the centre, 

 which is a little elevated above the poriferous surface, and the pores 

 are elongated clefts which pass under the central smooth place, so 

 that the pores of the one side are continuous with those upon the 

 other. The thin partitions between the pores penetrate some dis- 

 tance iuto the interior of the fossil. Figures g and 7c, plate iii. are 

 sections made through a specimen to shew this character. In nearly 

 all the rhombs one side of the smooth central space has an elevated 

 border. 



The arms are five in number, four of them extending down the 

 sides from the apex to the base; the fifth is a short arm, and reaches 

 only two or three lines from the summit on the left side (see pi. iii. 

 fig. E). They are composed of double series of joints alternating 

 with each other, and so loosely attached to the surface that they can 

 be easily removed with the point of a sharp knife. The ambulacral 

 grooves extend the whole length of the arms, and have on each side 

 a row of seven or eight pinnulae, those upon one side alternating 

 with those on the other. The two anterior arms unite a short 

 distance above the large rhomb. whi(;h is situated over the mouth 

 (see figs, a and n) ; their two ambulacral grooves then form but one, 

 and cross the apex to the posterior side (see ^g-g), where they divide 

 and run down the posterior pair of arms. In crossing the summit 

 the groove sends out a branch to the short arm, and, throughout 

 their length, small branches to the pinnule. 



The ambulacral orifice is very small, and situated in the bottom 

 of the groove, a little on the anterior side of that point where the 

 groove from the short arm enters the main apical furrow. This 

 aperture is usually concealed by the small marginal plates of the 

 ambulacra shewn in fig. g. Of these there is a row on each side of 

 the groove, and when the top of the fossil is so perfect as to exhibit 

 them well-preserved, they are always so firmly interlocked that they 

 completely close the furrow. In this state they no doubt formed a 

 securely protected covered way for the passage of the ambulacral 

 vessels to the arms. 



The minute aperture on the left side of the apex, near the ambu- 

 lacral orifice, is a minute pore situated in the centre of a sn^U 

 rounded tubercle (see fig. g). 



The pinnulae are scarcely one-third of a line in diameter, and 

 about half-an-inch in length. They are articulated in two series. 



