218 ROBERT TRACY JACKSON ON ECHINI. 



row; auricles exist as usually separate styles (text-fig. 227, p. 193): ambulacral tube-feet 

 dorsally are modified as ambulacral gills, and locomotion is effected in considerable part (A. 

 Agassiz, 1872, p. 264) by the spines. Any one of these characters might well be a case of 

 parallelism, but the sum of them gives strong evidence of kinship with the Exocycloida. I 

 therefore consider the Exocycloida as connected with the Arbaciidae, probably through some 

 early common ancestral stock. 



The suborder Holectypina is nearest akin to the Centrechinoida as it possesses compound 

 ambulacral plates and peristomal gills. The ambulacra are not petaloid dorsally as they usually 

 are in clypeastroids and spatangoids. The lantern is inclined with moderately deep foramen 

 magnum, and the wings of the pyramids are horizontally striated for attachment of inter- 

 pyramidal muscles as in regular Echini (Loven, 1892). The teeth are narrow, elongate, and 

 keeled, and the epiphyses are large, but narrow (Hawkins, 1909). These characters make a 

 near approach to those of the Stirodonta, but in part are like those of the clypeastroids. In 

 addition, in the Holectypina auricles exist on the ambulacra and there are low interambulacral 

 apophyses, both of which occur in the Centrechinoida, but auricles only exist in the clypeas- 

 troids. In the Holectypina the base of the corona is only partially resorbed so that one or more 

 primordial interambulacral plates are retained in the basicoronal row of the adult (Loven, 

 1874). In this suborder there are usually five genital plates with the madreporic pores in 

 genital 2, but in genital 5 there is no pore (text-fig. 171, p. 149), or genital 5 may be 

 absent. The test is usually circular and not specially modified as in the two following sub- 

 orders. 



In the suborder Clypeastrina the test is more or less flattened to discoid, circular, penta- 

 gonal, or other shape, and elongate and bilaterally symmetrical through III, 5. Internal 

 pillars and stalactitic growths are peculiar to the group, also lunules, or reentrant emargina- 

 tions may develop. Ambulacral plates are simple with the typical ambulacral pores limited 

 to a petaloid area dorsally ; ventrally there are numerous very minute pores, described by Mr. 

 Agassiz (1874, p. 695). They are so small as to be recognizable onlj- with difficulty. Both 

 the primordial ambulacral and interambulacral plates are in the basicoronal row (text-fig. 52, 

 p. 80) a character not known in regular Echini at any stage in life. Rarely (Arachnoides, p. 

 72, according to Loven, 1874), one interambulacral plate is pushed dorsally and the other four 

 are resorbed by intracoronal resorption so that the basicoronal row may be composed of ten 

 ambulacral plates only, a unique condition for Echini. The peristome is leathery without 

 plates, as far as I have been able to ascertain, a condition unknown in other Echini. Oculars 

 and genitals are fused into a mass; comparable to this is the partial fusion of genitals typical 

 of some spatangoids, and the partial fusion of genitals or genitals and oculars occurring as 

 variants in regular Echini (p. 167). There are four or five genital pores, but the posterior is 

 usually wanting. In Laganum diploporum, described by Messrs. Agassiz and Clark (1907), 

 curiously enough there are six genital pores, two being in area 5. The lantern is usually 



