NORTH AMERICAN STARFISHES. 



101 



Fig. 2.* 



.a« 



ai 



O..P 



the actlnal ring, showing the plate rising up on the side of tlie two 

 large ambulacral plates of the actinal ring. The 

 interambulacral plates (PI. XI 11. Fif/. 7) form small 

 scale-like plates near the base of the arm, carrying slen- 

 der pointed spines ; they increase somewhat, in size 

 at a distance from the base. They are followed on 

 the edge of the arm by two lozenge-shaped plates, 

 with extended points, carrying large club-shaped 

 spines forming a thin low wall for the support of the line of at- 

 tachment of the abactinal membrane covering the abactinal surface of 

 the arms. This membrane extends also over the central part of the 

 disk ; over the abactinal surface it is strengthened here and there by 

 a few small limestone plates or rods, placed at the base of the large 

 spines irregularly scattered on the surface of the abactinal region ; these 

 plates sometimes form in the disk a very irregular disconnected retic- 

 ulation, the lines of which are composed of small irregularly shaped 

 rounded plates. Within the space where the arms are united the am- 

 bulacral plates rise nearly vertically, but towards the extremity they 

 gradually slope more and more, inclining towards the actinostome, so that 

 the ambulacral plates form a hard flat area, occupying nearly the whole 

 of the actinal surface of the arms. 



The genera Pycnopodia and Crossaster are specially interesting on 

 account of the close relationship they have to Brisinga. In fiict, 

 comj^ared with Brisinga, they prove conclusively that the latter genus, 



* Fig. 2. — Profile view of actinal extremity of arm of Pycnopodia (the dermal covering of arm removed). 

 c'r, small plates forming the ridge, covering the ambulacral groove, 

 repeated along the whole length of the arm. a'c, the correspond- 

 ing plate of the second interambulacral plate. The upper projecting 

 part of this plate is the support of the basa! plate of the interbrachial 

 partition, and below it is seen the plate which strictly corresponds to 

 it. Tliis plate a'c also covers in part the plate n's of the basal am- 

 bulacral plate a', at the base of which is situated the interambulacral 

 plate a ip carrying the short sj)ines forming the moutli-papilhu. 



The relative position of the plates a's to the basal plate of the 

 interbrachial partition is well shown in Fit/. 3, which represents an 

 inside view, seen from above, of a part of the actinal ring, a's, a'c, 

 the same as in Fig. 2, ip, the interbrachial partition, reduced in this 

 genus to a few scale-like plates, supported on spurs of the interam- 

 bulacral plates, c', longitudinal groove, line of junction of ambulacral plates, c'r, ridge of connecting 

 plates forming groove c'. a o, opening for passage of ambulacral suckers. 



oCS 



