282 BULLETIN OF THE 



the adult, while those next the abactinal system are similar to the 

 younger stages. The plates of the poriferous zone increase indepen- 

 dently of the inter-ambulacral 'plates. The different stages of growth 

 represent in the younger stages Cidaris, next Hemicidaris, then 

 Pseudodiadema, Echinocidaris, Heliocidaris. The same general changes 

 take place in Toxopneustes lividus, but the turban shape (Cidaris 

 state) of the young test is more striking than in T. drobachiensis. 



In Cidaris the difference between old and young stages "is almost 

 entirely limited to the proportionally larger size of the spines, and 

 the more prominent serrations (recalling Salenocidaris). The abactinal 

 system early assumes the character of the adult ; in fact, with the excep- 

 tion of the smaller number of coronal plates, the above differences in 

 the spines are the only important changes undergone in this genus. 

 The same holds good for Diadema and Garelia, in both of which the 

 spines are proportionally larger, and being so much less numerous gives 

 to young Diadematidae a peculiar facies (D. calamaria-like). We find 

 also in young Diadema characters in the actinal membrane differing 

 from the adult ; the peculiar grouping, in five separate clusters, of the 

 buccal ambulacral plates which appear first, is soon lost by the en- 

 croachment of the smaller interambulacral plates, and in older speci- 

 mens the plates become deeply imbedded in the buccal membrane. 

 The pores at first are placed in a vertical row in very young speci- 

 mens ; they then become arranged in arcs of three or four pairs; with 

 increasing age the median rows of interambulacral tubercles assume 

 the arrangement found in the adult. Owing to the rapid growth of the 

 spines in the young, the extremity, and frequently the greater part of 

 the spine almost to the base, is hollow ; but as the young increase in 

 age they become more solid at the base, and further up in proportion 

 to their age.* Garelia is a good genus, as has been acknowledged by 



* The genus Echinodiadema of Verrill is founded upon structural peculiarities of 

 young Diadema mexicanum. Complete series of the young Diadema antillarum, from 

 one tenth of an inch in diameter upwards, show that : the slight cuts, the shape of the 

 abactinal system, the presence of small scales covering the anal system (few in number 

 in very small specimens), the trigeminate arrangement of the pores, the hollowness (gen- 

 erally upper extremity only) of the spines, due to the mode of growth and subsequent 

 solidification from the base upwards in Diadematidte, the arrangement of the tubercles, 

 the peculiar grouping of the plates of the buccal membrane, — features upon which the 

 genus has been characterized, — are found in young Diadematidae. I have carefully 

 examined the type of Mr. VerriU's species, as well as young of Diadema mexicanum, 



