DIALITHOCIDARIS. 55 



plates and one ocular pore, and the spines would similarly indicate in the 

 interambulacrum area one primordial plate and a row of two plates above it. 

 In the next stage of 1.5 mm. (including the spines) (Pis. 53, fig. G ; 54, fig. 1) 

 an additional primary spine has been added to each ambulacrum (Pis. 53, 

 fig. 6; 54, fig. 1). These spines (Pi. 53, fig. 7) are larger than those of 

 the interambulacral areas (PI. 53, fig. 6), and already have a well developed 

 milled ring. In this stage there are three pairs of ambulacral suckers 

 below the ambitus and as many above it, with a spherid at the actinal 

 edge of the ambulacrum. In tbis stage (1.5 mm.) it is difficult to decide if 

 the anus is covered with four anal plates. The keel of verrucae surround- 

 ing the anal system is somewhat irregular ; from the angles run branches 

 extending over the genital plates. 



The design of the abactinal system becomes quite regular in a somewhat 

 older stage of 2 mm. (PI. 54, fig. 4) ; the four plates of the anal system are 

 clearly defined and the radiating structure of the genital and ocular plates 

 is the same as that of an older specimen of 3.6 mm. (PI. 54, fig. 3). The 

 madreporic body and two of the genital pores of the trivium are fully 

 developed, while no trace could be seen of either in the specimen of 2 mm. 



The crowding of the interambulacral plates against the single actinal 

 primordial plates and its gradual diminution in height with increasing age is 

 well shown by comparing a part of the actinal system of a young Arbacia 

 of 3.G mm. (PI. 54, fig. :,') with its large pentagonal single actinal primordial 

 plates with a specimen of 16 mm. in which the second set of two plates has 

 been almost resorbed by the narrow primordial plate (PI. 54, figs. 5, G) ; the 

 extent of the pressure of the ambulacral plates in forming the multiporous 

 rows of ambulacral plates next to the actinal system is well seen on PI. 54, 

 figs. # and 5. 



DlALITHOCIDARIS A. Ag. 



Dialithocidaris A. Ag., Bull. M. C. Z. 1893, XXXII. No. 5, p. 75. 



The genus is marked by the great size of the genital and ocular plates, 

 the great width of the interambulacral areas, the linear arrangement of the 

 miliaries and sessile verrucse along the median line parallel with the hori- 

 zontal sutures of the upper interambulacral plates. The interambulacral 

 plates carry each two primary tubercles, the ambulacral, one. The sutures 

 of the abactinal interambulacral coronal plates are somewhat sunken and 

 bare. It has four anal plates like Coelopleurus. 



