THE TEST. 639 



extremely numerous, small, and though not differing in structure from those 

 of the regular Echini, yet are comparatively unimportant, and become still 

 less important in the Spatangoids, where in many places of the test the 

 tuberculatum is reduced to a mere granulation (PI. XV"., XXI".). In a few 

 genera of the three suborders, we sometimes find the scrobicular circles deeply 

 sunken, as, for instance, in Asthenosoma, where the miliary tubercles are 

 surrounded by a deep channel, formed by the scrobicular circles. In many 

 Clvpeastroids the scrobicular circles of the primaries are sunken ; in Echino- 

 neus (PL XIV".), and in some Spatangoid genera, — Breynia (PL XV". f. ?). 

 for instance. — where a few of the large tubercles within the peripetalous 

 fasciole have a similar structure. The large abactinal tubercles of Maretia 

 and Lovenia have a sunken scrobicular area ; but in Lovenia this is carried 

 to such an extent that the depression forms a purse on the interior of the 

 test (PL XXXVIII. f. 28). The calcareous test is covered by a thin cuticle, 

 crowded with vibratile cilia ; in the folds of the envelope and of the intestine 

 and other organs the plates forming the test, and the spicules found in the 

 walls of the intestine, are deposited (PL XXXVIII). These spicules are not 

 found in the Petalosticha. 



The ambulacral system forms, in the Clypeastroids (PL XP.f. 1) and Peta- 

 losticha (PL XXIII. f. 5), a bivium and a trivium. In the Clypeastroids the 

 ambulacra have all the same structure (PL XI'. f. 2). with the ex- 

 ception of differences of length in the petals (PL XIP.f. .;). and of the 

 lunules, placed in the prolongation of the petals, which are more or less well 

 developed in the different ambulacra, and are not materially different 

 (PL XIP.f. ■})■ The odd anterior ambulacrum forms, in connection with the 

 anterior lateral ambulacra, a trivium, in which the lateral anterior interambu- 

 lacra are placed symmetrically on each side of the median longitudinal 

 axis, and there is one odd median ambulacrum (PL XII'. f. .;). In the 

 bivium, on the contrary, the lateral posterior ambulacra are placed symmetri- 

 cally on the sides of the median longitudinal axis, and there is an odd 

 median interambulacrum (PL XII". f. .i). In Petalosticha the symmetrical 

 combinations possible in the trivium are complicated by the difference in 

 structure of the odd anterior ambulacral zone ( PI. XV. f. 2 ; PL XXI. f. .5). 

 It is, as I have attempted to show, impossible to trace a bivium and a trivium 

 in the Desmosticha which will be homologous to that of the Petalosticha and 

 Clvpeastroids, as there is no anterior or posterior developed ; what appears to be 

 the first trace of this in the regular Echini, the differentiation of a longitudinal 



