MUSEUM OF COMPARATIVE ZOOLOGY. 283 



Bolsche, in letters subsequently to the "Nachtrag" to his Diadematidaj, 

 in Wiegman's Archiv. Tlie spines are solid, already longitudinally stri- 

 ated in the youngest specimens examined, differing totally in their struc- 

 ture from those of Echinothrix or Diadema. This shows plainly that 

 in these embryonic Echini (Cidaridae, Diadematidae) the structure of 

 the spines forms a good basis for the discrimination of groups notwith- 

 standing their apparent great changes of form. These do not extend 

 to the nature of the ornamentation, which remains very constant, and 

 will prove of great value in fossil Echini. 



Nowhere among the young regular Echini have I found such great 

 changes in the shape and proportions of the test and spines as in Echi- 

 nometra. We frequently find specimens of the same size, where in one 

 case the outline is almost circular, the test flattened, covered with long 

 slender spines, while in the other the test is lobed, swollen, high, sur- 

 mounted by numerous short stout spines. These and all intermediate 

 stages, complicated by the greater or smaller number of primary 

 tubercles, the arrangement of the arcs of the poriferous zone undergoing 

 changes exactly similar to those described in Toxopneustes, are found 

 retained in specimens of very different size. This has given rise in a 

 great measure to the confused synonymy attached to our most common 

 species, and renders their identification, if based upon meagre material, 

 almost hopeless. 



In young Echinocidaridoa we have already in the youngest stages four 

 anal plates. The abactinal system of very young specimens is remarkably 

 prominent, occupying more than one half the abactinal part of the test. 

 The whole test is deeply pitted (Trigonocidaris-like) ; the rudimentary 

 tubercles, covering the greater part of the abactinal part of the test, are 

 connected "by ridges, which are gradually resorbed and reduced to the 

 granulation found upon the coronal plates of the genus. The primary 

 tubercles are at first limited to the ambitus, surmounted by short stout 

 spines (Podophora-like), gradually becoming more slender and propor- 

 tionally longer with increasing age (the opposite of what takes place in 

 Toxopneustes, Cidaris, and mo-t young Echini). The rudimentary 

 spines are not seated upon tubercles; they are club-shaped (identical 



of D. antillarum, and additional specimens of the so-called Echinodiadema coronatum, 

 which has convinced me that Verrill's species is only a young Diadema mexicanum, the 

 structural differences noticed being found in all young Diadematidce I have had occasion 

 to examine (D. antillarum, D. paucispinum, and I), mexicanum). 



