418 PKOF. F. J. BELL ON THE ECHINOMETRIDiE. [Mar. 15, 



Turning our attention now to the Echinidse, we may define them 

 as Regular Echinoidea, with external gills and five pairs of ambu- 

 lacral plates on the buccal membrane, in which some sets of primary 

 plates always fuse to form a secondary ambulacral plate, in which 

 the auricular arch is complete, and the rudimentary internal gill 

 entirely lost. 



If the above be, then, a good and fair definition of the Echinidse, 

 we come to a consideration of the points by which its constituent 

 genera may become grouped into distinct subfamilies. 



The tables already given show that it is hopeless to expect to be 

 able to find any ground of distinction on the absolute number of pairs 

 of pores in an arc ; we cannot say that, at any one point, forms with 

 three pairs of pores end and those with four begin. The character, not 

 being a constant or absolute one, is ui»fitted for use as & family-c\x3i- 

 racter ; nor are there any points which we can propose as affording so 

 wide a distinction between Echinometra and Echinus. Personal ob- 

 servation can only confirm the general tendency of the researches of 

 Perrier, Stewart, and Mackintosh on the histological characters of 

 the group in question, and lead to acquiescence in the conclusion of 

 M. Perrier : — " On le voit, les modifications qui caracterisent les 

 Echinometriens sont parfaitement nettes, mais ce ne sont que des 

 modifications dans le type des Echiniens. Le type ne change pas 

 comme lorsqu'on passe du Cidaris aux Diademes, et de ceux-ci aux 

 Echinocidaris ou aux Oursins proprement dits." 



These considerations appear to me to be sufficient to justify us in 

 Yei&mm^Echinometra, Strongylocentrotus, Echinus, &nAToxopneustes 

 in one family, and to refuse to follow Dr. Gray or Prof. Agassiz 

 in forming a family Echinometradse as distinguished from the 

 Echinidse. 



If we look yet a little further we shall find that the elaborateness 

 of the ambulacral plates, the strength of the spines, the size of the 

 buccal apparatus, appear to have culminated in Ileterocentrotus and 

 Colobocentrotus rather than in Tripneustes and Toxopneustes, which 

 in the latest Revision are, in the systematic list, placed furthest from 

 the Cidaridoe. 



Whatever be the significance of the obliquity of the morphological 

 axis, there can be but little doubt that it is of very great importance ; 

 and a return to the definition of " body circular," and to the recog- 

 nition of the differences insisted on by Johannes Midler, seems to 

 be better than a vague union of forms, elevated into a family for 

 no better reason than one that has already (p. 413) been quoted and 

 discussed. 



A scheme, therefore, of the following character will probably 

 throw into prominence the points of likeness and unlikeness in the 

 constituent members of the family Echinidse. 



Group I. Body circular ^ . . . . Echinin^. 



(a) Secondary plates formed of three 



primary plates e. g. Echinus. 



(ft) Secondary plate formed in adult 



