NORTH AMEKICAX STAIiFISIIES. 107 



tical areas, throu*rh which the water-tubes are protruded (PL XV^ Fifjs. 1, 2); 

 the network becomes closer towards the tip of the arms, and there are a 

 great number of small areas for the passage of the water-tubes (PI. X\'. 

 Fi(j. 3). In the living state the limestone skeleton is deeply imbedded in 

 a thick epidermis, completely covering the upper and lower surface of 

 the disk. Compare PL XV. Fifjs. \, V, with the figures PL XV. FUjs. 2, 3, 

 showing preparations of the plates of abactinal surface from the exterior 

 and interior. The interbrachial arches are reduced in this genus to a 

 mere vertical column, consisting in portions of not more than a single 

 plate placed close to the actinal ring, and leaving a large open space be- 

 tween it and the edge of the arm (PL XV. Fig. 6). 



The plates of the actinal floor form a regular pavement, diverging from 

 the interbrachial angle parallel to the axis of the arms; the actinal and 

 abactinal systems of plates form, at their junction on the edge of the arms, 

 a double row of large plates forming a binding at the periphery (PL XV. 

 Fig. 4), one row placed on the actinal side, the other on the abactinal 

 side (see PL XV. Figs. 4 and 2). There is a well-marked abactinal ori- 

 fice near the centre of the disk (PL XV. Fig. 1). 



In Gymnasteria, otherwise closely related to Asteropsis, there is no 

 special difference between the plates of the actinal and abactinal systems ; 

 they are more distinct, and not arranged quite so regularly as to form a 

 pavement. 



In the greater number of the pentagonal Starfishes we find the same 

 general distinction between the pavement-like plates of the actinal 

 side, extending to the junction of the actinal with the abactinal sys- 

 tem, although we do not always find so regular a peripheric series of 

 plates. This is the case in Culcita (see Figs. 4, 5). Whtre the actinal 

 plates acquire a great thickness, forming a lower floor through whicli 

 the passages between the plates and beams make an intricate system of 

 openings placed at different levels {Fig. 4), while the abactinal system is 

 reduced to a comparatively simple series of rods having the general ar- 

 rangement of triangular network with the longer or shorter rods separat- 

 ing them set on edge and imbricated (Fig. 5). The whole limestone sys- 

 tem is, as in Asteropsis, entirely imbedded in the thick epidermal iayer 

 in which the plates have been deposited, so that but a trace of the lime- 

 stone network appears when seen either in a natural condition or merely 

 in dried specimens. The interbrachial arch of Culcita is reduced to a few 



15 



