KAMPTOSOMA INDLSTINCTUM. 



Ill 



between the actinal and abactinal sides so marked. In the ainbulacral 

 area, from the actinostome to the ambitus (PI. 50, fig. 3) the primary 

 plates, ten in number, are irregular in outline, quadrangular, pentagonal, 

 becoming somewhat elongated towards the ambitus, with two wedge-shaped 

 poriferous plates between the second and third and the fourth and fifth 

 plates on one side, and between the fourth and fifth and sixth and 

 seventh on the other side. Between the seventh and m 



the eighth and ninth, small perforated lozenge-shaped 

 plates appear near the interambulacral edge ; these 

 tend towards the centre of the two rows of elongated 

 plates 1 which extend from the ambitus towards the ocu- 

 lar plates, becoming gradually higher as they approach 

 it (PI. 50, fig. &). There we find a series of pairs of 

 small plates each with a single pore, as have all the 

 primaries ami secondaries except the four primary 

 actinal plates, which have two disconnected pores 

 (PI. 50, figs. /, 3). As the plates increase in number 

 these pairs of small plates are forced down at the junc- 

 tion of two primary plates, Fig. 151, the larger of 

 the small plates finally occupying the central part of 

 the division line between the large ainbulacral plates ; 

 the smaller plate is pushed into the adjoining primary, 

 and its outline is lost between the fourth and fifth plates, 

 to become the single pore of the larger elongated primary plates. The 

 position of the wedge-shaped poriferous plates on the actinal side would 

 seem to indicate that the small persistent poriferous plate of the abactinal 

 side is in its turn, with one or two exceptions, sloughed off and resorbed 

 on the actinal side. In a specimen of this size, 42 mm., there are a large 

 number of primary interambulacral plates, — no less than eight from the 

 actinal system to the ambitus, and nine on the abactinal side (PI. 50, fig..?). 

 On the actinal side the primary ainbulacral plates carry only one or two 

 secondaries with a few miliaries irregularly placed. As the plates elongate, 

 and for four plates beyond the ambitus, they carry three to four secondaries 

 arranged in a horizontal row parallel to the sutures, with one or two inter- 

 calated miliaries. As the plates become higher nearer the ocular they 

 carry only one or two secondaries and as many miliaries. 



1 Wholly different in shape from any other ambulacra! plates in Eehinothurise. 



42 nun. 



Fig. 151. Kam ptosoma. 



