NORTH AMKIMCAX STARFISHES. 99 



Crossaster papposus. 



Crossaster j>tij>jiosiis M. T. 1810. Monat.sh. d. Ak:ul. IJcrl ii. 



PI. XI L 



In Crossaster the membrane covering thii abactinal system, like that 

 of Pycnopodia, forms a mere fihn, but it is strengthened by a regular 

 reticulation, with open meshes, carrying, at the points of junction of the 

 horizontal limestone plates, prominent club-shaped processes, upon the tip 

 of which are attached minute slender spines, forming more or less promi- 

 nent tufts (PI. XII. Fi//s. 3, 4). The interbrachial partition consists of a 

 membrane, without limestone plates, extending towards the base of the 

 arms, connecting the few limestone plates reaching from the actinal plate 

 to the abactinal surfiice with the triangle formed by the rising of the 

 actinal floor at the point of junction of adjoining arms (PI. XII. Fi(/. 3). 

 The actinal floor, with the exception of the plates of the interambulacral 

 area, is entirely composed of a compact pavement formed of small irregu- 

 larly shaped imbricating plates, gradually passing into the open reticula- 

 tions of the abactinal surface, along the sides of the arms (PI. XII. Fit/. 2). 



The ambulacral plates of this genus are broad and well separated; the 

 ambulacral groove is broad and prominent (PI. XII. Fi(/. 2); at the junc- 

 tion of the ambulacral and interambulacral plates the former are well 

 separated ; they are pointed, bulging in the central portion, leaving a 

 wide opening for the passage of the sucker. The basal plates take an 

 unusual development, forming a prominent ring round the actinostome ; 

 they are well separated by the interbrachial basal plates, forming the 

 base of attachment to the limestone plates, which constitute the basal 

 part of the interbrachial arch (PI. XU. Fiffs. 2, 3). The actinal side of 

 the interambulacral plates forms a series of slightly curved plates, at 

 right angles to the ambulacral groove, carrying tubervles diminishing 

 in size as they recede from the edge of the arms; these plates form a 

 prominent row along the edge of the arms on the actinal surface (PI. 

 XII. Fi(/. 2) ; the tubercles of the interambulacral plates, arranged in 

 narrow belts, carry slender spines, similar to those of the tut\s of 

 minute spines found on the abactinal surface. The basal interambulacral 

 plates, like their corresponding ambulacral plates, are immensely developed, 

 projecting far into the large actinal ring, and carrying, like all the 

 interambulacral plates, long, slender spines; these form powerful papilla?, 

 1-1 



