SEA-URCHINS. 471 



Clypeastroida is the presence in the latter group of a few large 

 ambulacrals in theadoral parts of the area, with no signs of phite- 

 crusliing. Can a compound plate be i-esolved by evolution into 

 . its constituent primaries, or is the simplicity of the Clypeastroid 

 ambulacral only apparent, being in leality the result of the 

 fusion of the components of a compound 2:)late, followed by the 

 atrophy of two of the three pore-pairs? Bather (59) has 

 expressed his belief in the possibility of the former process in his 

 discussion of the ambulacrals of Ortkopsis. In support of the 

 alternate suggestion it may be remarked that in the Clypeastroids 

 the pore-pair of each large pol3'gonal ambulacral is situated near 

 the adoral margin of the plate, leaving a high non-poriferous 

 region along the i-est of the adradial margin. Moreover, in tlie 

 case of Discoidea just cited, the triserial arrangement of the pore- 

 pairs is arrested soon after its inception, and the poriferous zones 

 again become straight. There seems to be no indication of 

 a corresponding reduction in the degree of compression of the 

 demi-plates towards the peristome, but rather an increase, until 

 the platelets become so minute that the small pore-pair can hardly 

 find room to pass through the test within its borders. The 

 presence of this feature of simplification in the sequence of the 

 pores, but not in the structure of the ambulacral plating, in 

 Discoidea seems especially significant ; for Discoidea is the nearest 

 ally of the Clypeastroids that is found among the Holectypoida. 

 However, I do not feel justified in expressing a positive opinion, 

 in one or the other direction, upon this question. Much must be 

 done in the study of the postlarval growth of the test in the 

 Clypeastroids before any proof of the origin of their ambulacral 

 structure can be expected. 



E. The I n t era m b u lacra, 



1. The Interamhidacral Plates. 



The interambulacral are always much broader than the 

 ambulacral areas, and the proportionate width (about 3 : 1 at the 

 ambitus) is retained almost unchanged from Pygaster to Conulus. 

 Owing to the absence of expanded petals and phyllodes in 

 the ambulacra, there is no compression of the adapical or adoral 

 extremities of the interambulacra such as occurs in most of the 

 Irregular Echinoids. The areas increase regularly in width from 

 the margins of the genital plates to the ambitus, and decrease 

 as regularly, though more rapidly, from the ambitus to the 

 peristome. 



In Holectypus depressus there are shallow pits on the transverse 

 sutures at points directly above the branchial slits. I know of 

 no evidence which could ascribe a function to such featui'es. 

 The interradial suture is usually only slightly zigzag in character, 

 and in some forms, notably among the Jurassic Holecliipi^ it is 

 practically straight, so that the plates become roughly rectangular 



