SYSTEMATIC CLASSIFICATION OF ECHINI. 217 



I, V, IV, II (text-figs. 128-153). Many species typically have the bivium insert, and one, 

 Strong ylocentrotus (jibbosus (p. 145), typically has an aberrant arrangement of oculars. Strongy- 

 locentrotus albus is peculiar in having frequent and also very rare forms of aberrant arrange- 

 ment of ocular plates (pp. 127, 162, 164). 



The Echinometridae, as here restricted, includes genera which are elliptical through a 

 sidewise axis, not through III, 5. As a secondary character, the ocular plates, when reaching 

 the periproct, do so in the sequence V, I, IV, not I, V, IV, as in the Echinidae and Strongy- 

 locentrotidae. This sequence is strongly held as shown in the table (p. 163), but occasionally 

 ocular I comes in first as a variant. These characters eliminate vStrongylocentrotus and its 

 allies, which have usually been included here, but which I include in Gregory's (1900) family 

 Strongylocentrotidae placed next to and associated with the Echinidae. The Echinometridae 

 besides the above have in some genera other striking and very specialized characters. In 

 Echinometra lucunter the auricles are extraordinarily developed as high spoon-shaped arches 

 (text-fig. 230, p. 193). In Heterocentrotus the primary spines attain excessive size and bulk, 

 and in another direction the spines of Colobocentrotus are most abbreviated, truncate; and in 

 C. atratus the spines are polygonal from mutual pressure, so as to resemble angular plates. 



The Exocycloida present a remarkable series of forms of Echini, all characterized by 

 having the periproct eccentric in interambulacrum 5. Assuming a monophyletic origin for 

 the group, the three sul)orders present a striking series of structural departures from the regular 

 Echini from which they doubtless originated. Considering the characters of the group as a 

 whole in brief, the compound ambulacral plates and peristomal gills of the Holectypina and the 

 auricles of that group and the Clypeastrina, the existence of keeled teeth, where teeth are 

 known, and the presence of sphaeridia, are all characters which unquestionably associate 

 the Exocycloida with the Centrechinoida and not with the Cidaroida, where these structures 

 are non-existent. Mr. Agassiz (1909) has shown that in the young of the spatangoid Echino- 

 neus a well developed lantern exists. This discovery is of the greatest interest and importance, 

 as previously teeth were unknown in this group. On examining his figures and description, 

 it is seen that the lantern is erect, teeth are keeled and epiphyses narrow, not extending over 

 the foramen magnum. Further, the interpyramidal muscles are short, the protractor and 

 compass muscles are inserted on the primordial interambulacral plates, and the retractors 

 are inserted on the auricles which are situated on the ambulacra. These details are shown 

 more fully by Westergren (1911), in a monograph on the genus Echinoneus, which death pre- 

 vented Mr. Agassiz from writing. Looking back to the Centrechinoida, we find that this type 

 of lantern exists only in the suborder Stirodonta. Further, the attachment of muscles, as 

 stated, occurs only in Arbacia and probably other members of its family. Looking at the 

 matter from the view-point of Arbacia, in that genus the teeth are keeled, epiphyses narrow 

 (text-fig. 212, p. 184); the primordial interambulacral plates are retained in the basicoronal 



