EHINOBRISSUS MICRASTEEOIDES. 67 
fascicle is broad, elliptical, and the greater part of the space it encloses is 
filled by the huge suckers of the odd anterior ambulacrum. The close 
resemblance of this stage of Hemiaster to an Acrope is very marked, and 
the permanence of the unusual development of the tentacles of this odd 
anterior ambulacrum up to the adult stage is an important link in tracing 
the affinities of such widely separated genera as Hemiaster, Brissopsis, Agas- 
sizia, Aerope, Aceste, and Schizaster. 
The structure of the larger interambulacral sj^ines in the young stages 
(7 mm. long, diameter) well shows the manner in which the fantastic shape 
of some of the Spatangoid spines is produced. The outer sheath of calcareous 
rods becomes solidified as thin lamellae, forming in one case in the primary 
interambulacral spines of the anterior part of the test on the abactinal side, 
above the ambitus, a spearlike head to the shaft of the radioles ; this may 
have a more or less lobed edge, or if the radiole is curved it forms a some- 
what shallow spoon-like extremity with spiny pi-ocesses ; in the shorter 
radioles of the actinal plastron the lamellge all develop into this spoon-shaped 
extremity, which may be perfectly symmetrical, or else developed une- 
qually on one side, according to the position of the radioles on the actinal 
plastron. 
In the earlier stages the fascioles are already covered by the same kind 
of pavement which we find in the adult, made up of short-stemmed, club- 
shaped spines closely packed together. There were in some of the larger 
specimens a few large, short-stemmed, globular pedicellarite, irregularly scat- 
tered over the abactinal surface of the test. 
Rhinobrissus micrasteroides A. Ac. 
Rhinobrissus micrasteroides A. Ac. Bull. M. C. Z., V,, No. 9, p. 192, 1878. 
Off Havana, 175 fatboiiis. 
Station 321, Lat. 32° 43' 25" N., Long. 77° 20' 20" W. 233 fathom.s< 
PL XXIII. Figs. 1-1,; PI XXVI Fig. I 
It is with considerable hesitation that this species is retained in the 
genus Rhinobrissus, the only specimen obtained by the Blake being a 
somewhat damaged young stage. From what is known of the modifica- 
tion of the Spatangoids due to growth, there are no characters in this 
single specimen which are not probably merely modifications due to age. 
The ambulacra are all flush with the test, and remind us of the earliest 
