THE TEST. 



The composition of the test in the three suborders of recent Echini is 

 readily reduced to a common formula, and, indeed, from an analysis of the 

 plates of the test alone we should hardly find sufficient data for a correct dis- 

 crimination of the suborders. The Desmosticha stand in marked contrast to 

 the other orders, owing to the large number of nearly identical coronal plates 

 which compose the test in the interambulacral space, extending from the 

 actinostome to the apical system (PL VI" '. f. 1 , 2) ; in the ambulacra! space 

 the arrangement of the plates is much more irregular; certain numbers of 

 plates form series, which are nearly repeated in vertical succession. The 

 plates of the ambulacral system are much more numerous in the Desmosticha 

 than those of the interambulacral spaces (PL VII f. 4), and in all Desmos- 

 ticha the test is more or less spherical, flattened, with a sort of ventral surface 

 (PI. VI a . f. 1) (the actinal surface), which forms an indistinct passage at the 

 ambitus (the edge of the test) to the dorsal side (the abactinal side). In 

 some of the regular Echini this passage is so gradual that no ambitus proper 

 can be said to exist (Amblypneustes, PL VHP.) ; while in others, such as 

 Colobocentrotus (PL III d . f. 5), some species of Pseudoboletia and Arbacia, 

 the contrast between the actinal and the abactinal side of the test is very 

 distinctly marked by the flattening of the test and the sharply defined 

 ambitus. 



In the Clypeastroids (Pl.XP.f. 1) the number of the coronal interambula- 

 cral plates is much smaller than in the Desmosticha ; the plates are of very 

 variable outline, the test being as it were built up of individual plates fitted 

 especially for their place. This specialization of the coronal plates is carried 

 to the greatest perfection in Petalosticha, and in some of the Brissina (PL 

 XXII.), Schizaster, and Moira (PL XXIII), and the like, the shape of the 

 plates is widely different in different parts of the test. In Clypeastroids we 

 find the greatest contrast between the actinal and abactinal side (PI. XIP. 

 f. 3, 4), — a contrast that extends not- only to the shape of the plates of both 

 the ambulacral and interambulacral areas, but to the intimate structure of 

 these two areas. The ambitus is well marked by a sharp edge ; the growth 



