PHORMOSOMA ri:.\M -. 35 



imbricated towards the actinaJ opening from their first appearance. It is 

 only when the young Phormosoma has attained a diameter of from 35 to 

 45 mm. thai the lapping of the coronal plates of the ambulacra! and inter- 

 ambulacral areas becomes distinct 



As in the young of other Echini the genital and ocular plates are uol 

 perforated in the youngest stages, nor can the madreporic genital be dis- 

 tinguished from the others, 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 readilj 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. I, 2), measuring 13 mm. in diameter, with these young stages of Phor- 

 mosoma (PI. XV. Figs. -'l-l!'). This comparison clearly brings out the strik- 

 ing difference between the structure of the actinal and abactinal systems 

 in the Diadematidse and Echinothuriae. In A.stropyga the genital ring 

 always remains closed, and the actinal system is provided with the ten 

 large buccal plate- characteristic of young Echinidae, and with an irregular 

 imbricating system of small imperforated plates forming a regular ambula- 

 cra! and an irregular intcramhulacral series. The peculiar forked hand of 

 pigment cells of the abactinal part of the coronal plate- so characteristic 



of A-tropvja seems to he the remnant of the pigment spot- which are 

 irregularly scattered over the test of young Kchinothurhe. and which in the 

 Diadematidse 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 thiekh covered with large prominent patches of pigment. 



Phormosoma uranus Wyv. Thome 



Phormosoma Petersii A. A.;. Hull M < !. Z , Mil.. No. -'. p. 76, 1880. 

 I., ei Am Hi . Lit. 39 IV W" N.,] W 399-1224 fatho 



/'/. A'.. /'/. XI. 



Thomson figured in tl Voyage of the challenger" (I., p. 1 Hi. Fig. •"■■"'• 



p. 117. Fig. 34) a small specimen of Phormosoma which he called /'. 



It differed very strikingly from specimens of a closehj allied Bpecies collected 



by the 15lake.whi.di I had provisionally named /'. /'- ersU. The principal 



