SHALLOW-WATER STARFISHES 



37 



together and usually bearing six oral spines. The four terminal 

 spines (peroral spines or papillae} are directed more or less hori- 

 zontally over the mouth (a, of} ; the two outer ones (a') are diver- 

 gent and smaller and are sometimes lacking. The outer angle of the 

 jaw, formed by the second pair of adambulacral plates, bears an erect 

 and usually longer pair of (epioral) spines (e, e). 



The characters of these oral spines and plates often afford good 

 specific distinctions, and in some cases at least are of generic value ; 

 but they have not yet been described or figured with care, except in 

 a comparatively small number of species, so that at present their 



11 



FIG. 3- 



Orthasterias calif arnica V ., type, i, A jaw; a, a; a', a', apical or peroral spines with pedi- 

 cellarix (p, p); e, e, epiorals; e', odd epioral; X 15; ii, an adambulacral spine. X 15. 



value is somewhat uncertain. In general characters they are remark- 

 ably constant throughout this family, and in species that differ widely 

 in other respects. This would indicate that their special arrangement 

 has persisted for a very long period of time, for it must have been 

 established before the world-wide dispersal of the family took place, 

 and probably before the family itself was differentiated. 



In the calcareous peristomal ring the jaw-plates, formed by the 

 union of two ambulacral plates, are usually more prominent than the 

 plates alternating with them, known as the odontophores, formed by 

 the union of two interambulacral plates. Such an arrangement is 



