184 HAWAIIAN AND OTHER PACIFIC ECHINI. 



ambitus and 27 in the ambulacral column. Between the abactinal system 

 and the ambitus there are 21 plates in each half of an interambulacrum and 

 38 in an ambulacrum. At the ambitus the interambulacral area has a 

 width of 42 mm., the ambulacral, 32. The greatest diameter of the 

 abactinal system is 21 mm., that of the actinal system 30 mm. The 

 primary spines both on the actinal and abactinal surfaces are sharp and 

 slender, from 12 to 22 mm. in length (PI. 76). The longer spines of the 

 actinal side are slightly expanded at the tip (PI. 76,- fig. 2), and terminate 

 in a small white "hoof" which is remarkable for being thickest at base 

 and nearly pointed at tip. Between the actinal interambulacral plates 

 there is even more uncalcified membrane than in A. fenestratum. On the 

 abactinal side the calcification of the plates increases gradually from 

 below the ambitus to the abactinal system (PI. 76, fig. 1). Both the am- 

 bulacral and interambulacral plates are higher in fenestratum than in 

 leptaleum. 



On the actinal side the vertical row of interambulacral primaries border- 

 ing the ambulacrum is very marked, and extends just over the ambitus to 

 the abactinal surface, close to the ambulacral plates. On the abactinal 

 surface two irregular series of smaller primaries extend on each half of the 

 interambulacrum, two-thirds of the way to the abactinal system (PI. 76, 

 fig. i). The two median rows of ambulacral primary tubercles are distant 

 and irregular (PL 77, fig. 3), but extend from the actinostome (PI. 77, 

 fig. l) over the ambitus (PI. 77, fig. 4) nearly to the abactinal system 

 (PI. 76, fig. 1). The two inner series of pairs of pores are well separated 

 from the outer row for nearly the whole length of the ambulacrum 

 (PL 77, fig. Jf), approaching closely only near the actinostome (PL 77, 

 fig. 3). 



There are from ten to twelve rows of rather high imbricating porif- 

 erous plates extending from the teeth to the coronal, ambulacral plates 

 (PL 77, fig. 1). They each carry a horizontal row of small secondaries and 

 miliaries. The small area between the ambulacral plates at the proximal 

 margin of the primary interambulacral plate is covered with a few minute 

 elliptical plates. The actinal plates near the teeth all abut on each other 

 as regularly as the coronal plates of the typical echinoid test ; it is only 

 the plates near the coronal plates which are imbricating. The median 

 suture of the interan\bulacral area extends almost unbroken from the 

 actinal edge of the test to the teeth, and the adjoining ambulacral areas 



I 



