THE GENUS COLOBOCENTROTUS. 27 



side is found a set of three or four layers of yoke-shaped spicules (Pis. 16, fig. 

 6; 45, fig. 8). In P. jJedifera the yoke-shaped spicules are nearly straight 

 (Pi. 16, fig. 8), with a row of small perforations on the under side of the 

 spicule. In P. atrata the yoke-shaped spicule is well arched (PL 16, fig. 12) 

 with but few larger perforations. 



These perforations frequently pass into more or less prominent teeth (PI. 

 45, figs. ^ 6). The yoke-shaped spicules of Coloh. Stimjjsoni (PI. 45, figs. 

 3-6) and Mertensii (PL 45, figs. 8, 9) have fewer perforations than those of 

 P. pedifera, but more than those of P. atrata. The position of the layers 

 of the yoke-shaped spicules with reference to the basal disks is well seen in 

 PL 16, figs. 6, 9, 10, and PL 45, fig. 8. Fig. U, PL 16, and fig. 10, PL 45, 

 show the atrophied tip of an abactinal pedicel. No spicules were found in 

 the tubes of the pedicels. 



The changes due to growth in the appearance of the basal disk are shown 

 in figs. 11 and 13, PL 16 ; the former is the basal plate of the pedicel of a de- 

 nuded specimen of P. atrata measuring 8 mm. in diameter, the other of a 

 pedicel from a specimen measuring ,45'mm. with spines. -y A 



The basal disks of the other species were taken from specimens varying / 



between 70 and 88 mm. in diameter including the spines ; P. pedifera, 82 mm. ; 

 Colob. Mertensii, 70 mm. ; Coloh. Stimpsoni, 88 mm. 



The Pedicellari^ of Colobocentrotus and Podophora. 

 Plates 3" ; 4-6 ; 17-19 ; 29 ; 30. 



The four kinds of pedicellariae of Podophora joedifera are figured on 

 PL 3% figs. 1-4. The rods vary but little in length, but the length of the 

 necks differs greatly. In the tridentate pedicellarige (PL 3% fig. 1) the neck 

 is nearly twice as long as the rod ; in the ophicephalous pedicellariae (PL 3^, 

 fig. 2) it is but slightly longer than the rod. 



The heads of the triphyllous and globiferous pedicellariae are small (PL 3% 

 figs. 3, 4), that of the triphyllous pedicellarise being scarcely wider than the 

 neck ; this is about twice the length of the rod (PL 3% fig. 3), while the neck 

 of the globiferous pedicellarige is reduced to a minimum, the valves seeming 

 to rest upon the head of the rod (PL 3'', fig. .4)- 



The heads of the tridentate pedicellariae of Podophora atrata seem 

 to be somewhat more elongated and pointed (PL 3*, fig. 5) than those of 

 P. pedifera, and have a much shorter neck. The ophicephalous pedicellariae 



