96 PANAMIC DEEP SEA ECHINI. 



in old specimens a flow of the anal plates into the interambulacrum, similar 

 to the flow of the ambulacral plates of the corona onto the buccal plates 

 of the actinal system. The figures which I here give of the young of 

 Phormosoma hisjndum show plainly that I was mistaken in my observa- 

 tions on the mode of formation of the buccal plates of the actinostome of 

 Echinothuriae. I wish here to state that the plates illustrating this point 

 were drawn and printed several years before Dr. Mortensen ' called atten- 

 tion to the error. 



In the largest specimen collected, 203 mm. in diameter (Pis. 37; 38), 

 the abactinal system (PI. 39, fig. 8) was 44 mm., and the actinal system (PI. 

 41, fig. .') 46 ram. The three interambulacral plates nearest the ambitus 

 (PI. 49, fig. 5) carry two large primary tubercles occupying each nearly the 

 whole width of the inner and outer parts of the plate ; the next plates have 

 only one much smaller primary tubercle on the outer part of the plate. 

 The rest of the interambulacral plates is fairly covered by secondaries and 

 miliaries; they are more closely packed in the plates above the primordial 

 one. In the actinal system (PI. 4 1, fig. ..') there are seven rows of buccal 

 plates well covered by an actinal row of secondaries with a, few intercalated 

 miliaries. 



The large, primary radioles near the ambitus on the actinal side are 

 slightly curved, and end with a shoe-shaped bevelled tip having a slightly 

 curved edge (PI. 45, figs. 16-18). The radioles of the abactinal side are 

 straight, somewhat pointed. The difference in the size of the primary 

 radioles of the abactinal and actinal sides of the test near the ambitus 

 becomes very apparent in specimens of 75 mm. (PI. 31, figs, y, ,.'). In 

 specimens of 120 mm. and over the contrast is still greater (Pis. 31, fig. S; 

 35; 36), and in those of 134 mm. fully as marked as in the largest speci- 

 mens collected, 203 mm. in diameter (Pis. 37 ; 38). 



The breaking up of the interambulacral and larger ambulacral plates 

 can be seen in Plates 37 and 38. The general aspect of this breaking 

 up is also shown in Plate 38 on the plates nearest the ambit us of the 

 lower part of the figure. The splitting of the plates already occurs in 

 specimens of 40 mm. (PI. 30, fig. 8), and is well seen in Plate 30 fig. 10 

 in the left interambulacral areas of a specimen 55 nun. in diameter. On 

 Plate 32, fig. 1, the breaking up of the plates of the odd and left posterior 

 interambulacra, seen from the abactinal side, is very distinctly shown. In 



1 Mortensen, 1. c. p. 171. 



