﻿194 ASTKOPIIYTON COSTOSUM, 



bear the arm-spines, in the form of single microscopic hooks. These 

 hooks seem only to be found on the fine twigs. Disk, above and below, 

 covered with the same granular coat as the arms ; above, also, are 

 found the round spots in the interbrachial spaces. Radial ribs very 

 large and prominent, ending without in a smooth, concave cicatrix ; 

 on their upper surface are numerous u-regular spines, about 1.5""" long, 

 with deeply striated or fluted sides. Interbrachial spaces below very 

 small, in consequence of the great breadth of the arms. Genital slits 

 small, without margin of grains ; length, 4.5°"°'. Tentacle-scales very 

 fine, slender, spiniform ; they are not found at all within the ninth fork 

 of the arm on its main trunk ; beyond this point there are one, two, 

 three, and even four to each pore, the number being greatest on the 

 smaller side branches ; on the fine twigs, however, the number de- 

 creases, and the finest have them replaced by a single hook. Color, in 

 alcohol : above, a fine purplish umber, with fine, indistinct spots of 

 lighter ; below, pale umber. 



Variations. — Instead of being uniform, the upper surface is often 

 spotted with darker. The teeth in younger specimens are often com- 

 paratively shorter and blunter, and the spines on the radial ribs are 

 smooth, and not so numerous as in the adult. The young of this spe- 

 cies have, on the basal part of the arm, a few short spines or large 

 grains. The radial ribs are low and faintly marked, and bear at their 

 outer end a large spine, like a horn ; besides these there are only one 

 or two small spines on the disk ; the first fork of the arms is not at the 

 margin of the disk, but a little way from it ; on the under side of the 

 arms the tentacle-scales, two in number, extend inward as far as the 

 first fork. A small specimen had a disk of 10°"°-, and twelve forks to 

 the arms, as follows : — 



Disk to first foi-k 3.0"""- 



First fork to second, 4.0 " 



Second " " third, G.O " 



Tliird " " fourth G.O " 



Fourth •' '• fiftli 4.0 " 



Fifth " ■' sixth, 4.0 " 



Sixth " " seventh, 6.0 " 



Seventh " " eighth, 4.5 " 



Eighth " " ninth, 4.5 " 



Ninth " " tenth, 4.5 " 



Tenth " '• eleventh 4.5 " 



Eleventh " " twelfth, 5.5 " 



Twelfth " " end, 3.0 " 



60.0"""- 



Astroi^hyton costosum has been found in the West Indies (Orsted, 

 Eiise, Suenson, Benzon), on the coast of Florida, at the Tortugas Isl- 



