240 HAWAIIAN AND OTHER PACIFIC ECHINI. 



The arrangement of the ocular plates with reference to the periproct, a 

 morphological character so beautifully worked out in Jackson's recent mono- 

 graph (1912, Mem. Boston Soc. Nat. Hist., VIL, p. 8G-164) is of great impor- 

 tance in tracing the lines of development in the Echinidse, but it is impossible to 

 rely on it alone. The same is true of the amount of calcification in the buccal 

 membrane, the relative sizes of the spines and tubercles and the characters of 

 the globiferous pedicellarise, though all of these are of the greatest help. Finally 

 it may be emphasized that, as Mortensen has pointed out, color is often a sug- 

 gestive character, and although in certain species (as Echinus acuius and Lytechi- 

 nus variegalus) it is very variable, in most cases it affords a good deal of assistance 

 in determining specific hmits. 



Having thus indicated the characters in which the specific differentiation 

 of the Echinidse is best shown, it is desirable to point out what seem to have been 

 the lines of development and the resultant, most highly specialized genera. 

 There can be little question that the two species of Psammechinus {Echinus 

 miliaris Gmel. and E. microluherculatus Bl.) are the least speciaUzed members 

 of the family, at present known. This is shown by the uniform series of am- 

 bulacral plates, the small exsert oculars, the absence of distinct gill-cuts, the 

 heavily plated buccal membrane, the slight differentiation of primary spines 

 and tubercles, and the character of the globiferous pedicellariae. From some 

 such stock, the species of Echinus have undoubtedly come and it is not difficult 

 to trace possible lines of differentiation. It seems probable that Alexandri 

 is not far from the ancestral stock, as shown by the ambulacra and abactinal 

 system, though the buccal membrane, the primary spines, and the globiferous 

 pedicellaria?, all show considerable specialization. Although Wallisii, atlanticus, 

 and gracilis all show a very high degree of specific differentiation, it seems likely 

 they originated from the Echinus-stock near Alexandri. Echinus acutus is the 

 most highly variable species of the family-, and while some of its forms arc very 

 near Alexandri, it shows tendencies towards specialization in several different 

 directions. The resulting extremes are so utterly unlike, one would never 

 consider them conspecific were it not for the completeness of the intermediate 

 series. From acutus as a centre, such species as esculcntus and tenuispinus have 

 arisen on the one hand l)y marked changes in the ambulacra without accompany- 

 ing changes in tlio aliactinnl systoin. while on the other hand ftiargaritaceu^ has 

 developed with less specialization of the ambulacra but with greater changes 

 in (lie abactinal system. From similar stock the develoi)ment, through E. 

 armatus, of Evcchinus, the most specialized genus of this branch of the family, 



