212 



PANAMIC DEEP SEA ECHINI. 



I am able to give figures of the plates of young Abatus 1.6 nun. in 

 length kindly sent me by Mr. Rathbun of the Smithsonian. They are 

 perhaps the most Echinid-like post-embryonic stages as yet figured of any 

 Spatangoid, Figs. 306-308 (PI. 99, figs. 3-7). 



In the young Echinocardium figured by Loven the anal system is fully 

 developed in the abactinal part of the posterior ambulacrum, and the actinal 

 sternum has already assumed the characters of the adult sternum. This 

 is in great contrast to the stages of Abatus I am able to describe. The 

 young, when seen from above or below, is but slightly elliptical (PI. 99, 

 figs. 8, 4), the actinostome is pentagonal, somewhat excentric, the interam- 

 bulacra are all in contact with it, though in older specimens (PI. 99, fig. ,s') 

 the posterior lateral ambulacra are excluded from the actinostome. The 

 sternum consists of two plates of very different dimensions forming almost 

 a true meridosternum (PI. 99, figs. S, 5), which in the adult becomes 

 amphisternal (PI. 99, fig. 5). 



The anal system of the young Abatus, Fig. 306, is not as yet separated 

 from the abactinal system by the abactinal plates of the odd interambulacral 



area (PI. 99, fig. 5). The two systems 

 are enclosed in the same area within the 

 circle of the ocular plates (PI. 99, figs. 

 4, 6, 7), as has already been figured by 

 Loven. 1 Loven's figures are from a 

 specimen somewhat larger than mine • 

 the interambulacral plates are not quite 

 as far advanced. The fascioles are more 

 marked in my figures (PI. 99, figs. £, 

 4, .5), the interambulacral spines some- 

 what larger, and the suckers more prominent (PI. 99, fig. l). Only few of 

 the spines are curved as they appear in somewhat older specimens 2 which 

 I figured on a former occasion. 3 



In the smallest Abatus (PI. 99, figs. 4, 6), Fig. 307, the anal system is 

 indicated by a nearly bare membrane, while in a somewhat older specimen 

 1.9 mm. in length, Fig. 308, the anal system appears towards the posterior 

 edge of the abactinal area surrounded by a row of radiating plates (PI. 99, 

 fig. 7) identical in number with those of the adult (PI. 99, fig, 10), Fig. 309. 



3 mm. 



Fig. 306. Abatus cavebnobus. 



1 Pourtalesia, PI. XIV, fig. 164. 



8 Proc. Am. Acad. 1876, p. 231. 



" Challenger " Echinoidea, PI. XX a , fig. 8. 



