HAWAIIAN AND OTHEE PACIFIC ECHINI. 



Collected by the U. S. Fish Commission Steamer Albatross, Commander 

 Chauncey Thomas, U. S. N., Commanding in 1902, and Lieut. Com- 

 mander L. IM. Garrett, U. S. N., Commanding in 1906. 



CLYPEASTRINA Gregory. 

 General Characteristics. 



Although closely related to the now extinct Holectypina, the suborder 

 Clypeastrina is sharply marked off from all recent Echini by the form of the 

 test, the position of the periproct, the presence of jaws and auricles, in the absence 

 of peristomal gills, and the usually petaloid arrangement of the ambulacra 

 aborally. Such a combination of characters gives them a general appearance 

 which is usually easily recognizable and as they all resemble more or less closely 

 the well-known species of the typical genus Clypeaster, they have come to be 

 colloquially known as clypeastroids. The group is not only well defined and 

 homogeneous but is a relatively small one and of comparatively recent geological 

 appearance. 



There is good ground for the belief that they arose in connection with the 

 Holectypina from a stirodont group of regular Echini, of which the Arbaciidae 

 are modern representatives. The evidence in support of this belief has been 

 briefly set forth by Jackson (1912, Phylogeny of the Echini. Mem. Boston Soc. 

 Nat. Hist., 7, p. 217). But it ought to be pointed out that the primary tubercles 

 in all clypeastroids are perforate, while this is true of no Stirodonta save some of 

 the fossil Saleniidae. This may be interpreted however, simply as the retention 

 of an ancestral character, for it is found in all the primitive regular Echini. 



The classification of the clypeastroids is based primarily upon the auricles, 

 with which the well developed but characteristic lantern is associated. In the 

 more typical and primitive members of the group, the auricles of each ambula- 

 crum are not only distinct but are well separated from each other. In more 

 specialized forms the right hand auricle of one ambulacrum and the left hand 

 auricle of the adjoining ambulacrum have become more or less completely trans- 

 posed on to the separating interambulacral plate and are more or less fused into a 



