NORTH AMERICAN STARFISHES. 109 



part of the spines scattered over the abactinal area, leaving but a short 

 piece of the end of the spines bare. The meshwork of course becomes 

 closer towards the extremity of the arms ; the plates and rods are of 

 considerable thickness, as is seen in their section along the edge of the 

 arm (see Fig. 5, PI. XVI.). 



The junction of the actinal and abactinal systems forms a double row 

 of large contiguous plates carrying a heavy spine (see Figs. 4, 5, PI. XVI.). 

 The pavement-like plates of the actinal surface are arranged in rows 

 parallel in a general way to the longitudinal axis of the arms (PL XVI. 

 Fig. 4), and also in indistinct rows at right angles to this (PI. XVI. 

 Fig. 2). They are well covered by a coarser granulation than that of 

 the abactinal surface, the central part of the plate carrying a cluster of 

 three to five larger granules, becoming in some cases nearly fixed spines ; 

 these granules, on the actinal surface of the interambulacral plates, become 

 a large flat pointed movable spine, with smaller flat lateral spines, rounded 

 at their extremity. Round the edge of the interambulacral pieces forming 

 the jaws they increase materially in size, becoming very prominent mouth- 

 papilla) (PI. XVI. Figs. 2, 3). 



In all the pentagonal Starfishes the fact that the jaw pieces are simply 

 the modified interambulacral plates of the last joint is very apparent, as 

 well as that the interbrachial plates forming the base of the interbrachial 

 arch are also only a modified part of the interambulacral plates formed 

 by the soldering together of the inner lateral spaces of the opposite 

 interambulacral plates of the joint of the jaw. 



The interbrachial arches are composed of comparatively few large solid 

 plates ; their breadth varies materially in different specimens, either nearly 

 filling the whole space between the actinal ring and the angle of the 

 arms, or limited to a shorter wall next to the mouth. The ambulacra! 

 system is composed of tall plates rising well above the actinal floor, 

 forming a broad median groove, seen from the abactinal side (PI. XVI. 

 Fig. 5) ; when seen in profile (PI. XVL Fig. 7), large elliptical spaces are 

 left for the passage of the powerful ambulacral suckers (PI. XVI. Fig. 2). 

 The interambulacral plates are large, distinct, and of great thickness, with 

 their actinal face well developed (see Figs. 4, 7, PI. XVI.) ; the last joints 

 of the plates of the actinal ring are prominent, raised high above the 

 interbrachial plates. The jaws are large, projecting far into the central 

 actinal space (PI. XVI. Figs. 4, 5, 7); the papilla) when extended meet, 



