ARBACIA PUNCTULATA. 265 



The South Carolina specimens are usually brick-red color in the bare inter- 

 ambulacral spaces with darker sutures, the spines are tipped of same color, 

 but fade gradually towards base into a light-colored violet, or even pale 

 yellow. The tentacles are yellowish-white ; the few pedicellaria? found upon 

 test (upper part) are of a dull white color. 



In the youngest Arbaciadae observed, we have already four anal plates 

 (PL V. f. 9). The abactinal system of very young specimens is remarkably 

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

 (PL V.f. 11.) The whole abactinal part of the test is deeply pitted, Trigo- 

 nocidaris-like (PI. V.f. n); the rudimentary tubercles, covering a part of 

 the abactinal part of the test, are connected by ridges (PI. V.f. 11), which 

 are gradually resorbed and reduced to the granulation found upon the coro- 

 nal plates of this genus. The primary tubercles are at first limited to the 

 ambitus, surmounted by short stout lanceolate spines, Podophora-like (PL V. 

 f. 9), gradually becoming more slender and proportionally longer with in- 

 creasing age (PI. V.f. 12, 13, 17), — the opposite of what takes place in Stron- 

 gylocentrotus, Cidaris, and most young Echini. The rudimentary spines 

 are not seated upon tubercles; they are club-shaped, somewhat similar in 

 shape to those of Podocidaris. The poriferous zone has in the earliest 

 stages the structure found in the adult, only it does not widen at the acti- 

 nostome (PL V.f. 15). The ratio of the actinostome to test does not vary 

 greatly in different stages of youth ; the edge of the actinal system forming 

 the groove of the gills is turned back but slightly in young, the lips taking 

 the place of cuts becoming more prominent with increasing age. The sepa- 

 ration of Agarites and Tetrapygus to represent the groups with bare or 

 crowded interarnbiilacra is not natural, depending upon the greater or less 

 resorption of the rudimentary tubercles formed in the earlier stages. It is 

 very common to find young of Arbacia punctulata which would pass for 

 young of Tetrapygus, and young Arbacia pustulosa which would pass for 

 young Agarites. Owing to the independent growth of the plates of the 

 poriferous zone, we have either three or four pairs of pores for each ambula- 

 cral plate ; the same is the case with other Oligoporidae, as limited by Desor, 

 showing that the division he has made, convenient though it is as a key for 

 the easier grouping of genera, is yet not strictly reliable, the mode of 

 growth of many Polyporidae showing in their young stages that they have 

 but a small number of pores for each ambulacral plate, which places them 

 among the Oligoporidae ; but, owing to the independent growth of the plates 



