fi38 THE TEST. 



nf the two surfaces proceeds independently after reaching a certain size ; the 

 character of the spines of the two surfaces is usually different; but the most 

 marked difference exists in the structure of the ambulacral system. which forms 

 in all Clypeastroids ambulacral furrows, ramifying more or less over the actinal 

 surface, the thin, delicate water-tubes of which are in striking- contrast to the 

 comparatively powerful ambulacral tentacles of the abactinal surface. As in 

 the Petalosticha the number of ambulacral plates of the Clypeastroids is 

 greatly reduced on the actinal side, and immediately below the petals they 

 do not exceed in number the Lnterambulacral plates. The interambu- 

 lacra do not extend to the actinostome, but the ambulacra! plates encroach 

 upon them, forming what has been called the buccal rosette, immediately 

 round the mouth (PI. XIP.f. .;); this rosette consists of ambulacral plates, 

 it precedes the interaiubulacral area, commencing with a single plate. The 

 contrast between the actinal and abactinal side is not so sharply marked in 

 Fibularia and the Echinanthidae | PI. XI b .) as in the Scutellidae (PI.XIP.), 

 Laganidae, and the like, in which the most characteristic structural features 

 of the suborders are fully developed. There are in the Clypeastroids but 

 few variations in the structure of the abactinal part of the ambulacral sys- 

 tem. The great structural distinctions are mainly found on the actinal side. 

 In the Petalosticha the specialization of the interaiubulacral plates is carried 

 to its height. The actinostome also presents some very characteristic dif- 

 ferences ; the outline formed by the vi\^v of the test is subject to consider- 

 able, changes [Pis. AT.. XVII., XXI, XX//L XXIII ". i. In the other 

 suborders, especially in the Clypeastroids, the uniformity of the actinal open- 

 ing is very marked. The Petalosticha are distinguished from the Clypeas- 

 troids by the formation of plastrons (Pis. XXI f. >,.:,; XXI'.f.s) where 

 the tuberculatum is usually closely packed,,often surrounded by bands formed 

 of minute miliaries (the fascioles) ; but what is especially characteristic of 

 this suborder are the remarkable changes 1<> which the abactinal part of the 

 ambulacra! system is subject in the different families, and the specialization 

 of one of the ambulacra, which in many genera shows a completely different 

 structure from that of the Lateral ambulacra i /'/. A" V". f. i.r; -XIX".. 

 A/A'.. XX /.. XXIII".). The tubercles which form so conspicuous a part of 

 the test of the regular Echini, as in the case of the Cidaridae ( PI. 11. f. l). 

 where they are limited in number and very large, are gradually reduced in 

 importance as we pass through the Diadeinatidae [VI. III.". IIP.) to the 

 Echinidae (PI. VHP.. VHP.), until in the Clypeastroids (PI. XI') they are 



