PHOEMOSOMA UEANUS. 35 
imbricated towards the actinal opening from their first appearance. It is 
only when the young Phormosoma has attained a diameter of from 35 to 
45 mm. that tlie lapping of the coronal plates of the ambulacral and inter- 
ambulacral areas becomes distinct. 
As in the young of other Echini the genital and ocular plates are not 
perforated in the youngest stages, nor can the madreporic genital be dis- 
tinguished from the othei's, owing to the similarity of the calcareous meshes 
of the young genital plates to the openings of the madreporic canal. In 
somewhat older stages, however, the madreporic genital is readily distin- 
guished from the others (in a specimen measuring 20 mm. in diameter). 
This is also the stage at which the other plates of the abactinal system 
begin to show the perforations for the genital and ocular pores. 
It is interesting to compare the structure of a young Astropyga (PI. XV. 
Figs. 1, 2), measuring 13 ram. in diameter, with these young stages of Phor- 
mosoma (PI. XV. Figs. 3-19). This comparison clearly brings out the strik- 
ing difference between the structui'e of the actinal and abactinal systems 
in the Diadematida3 and Echinothurioe. In Astropj'ga the genital ring- 
always remains closed, and the actinal system is provided with the ten 
large buccal plates characteristic of young EchinidEe, and with an irregular 
imbricating system of small imperforated plates forming a regular ambula- 
cral and an irregular interambulacral series. The peculiar forked band of 
pigment cells of the abactinal part of the coronal plates so characteristic 
of Astropyga seems to be the remnant of the pigment spots wliich are 
irregularly scattered over the test of young Echinothurite, and which in the 
Diadeniatida3 have a regular arrangement. These large pigment spots are 
eminently an embryonic character, for we find the test of the young stages 
of regular Echini thickly covered with large prominent patches of pigment. 
Phormosoma uranus Wtv. Thoms. 
Phormosoma PetersU A. Ag. Bull. M. C. Z,, VHI., No. 2, p. 76, 1880. 
Lesser Antilles, — Lat. 39° 45' 40" N., Long. 70° 55' W. 399-1224 fathoms. 
PI. X., PL XL 
Thomson figured in the " Voyage of the Challenger " (I., p. 146, Fig. 33, 
p. 147, Fig. 34) a small specimen of Phormosoma which he called P. uranus. 
It differed very strikingly from specimens of a closely allied species collected 
by the Blake, which I had provisionally named P. Pdeisii, The principal 
