104 THE VOYAGE OF H.M.S. CHALLENGER. 



Among tlie small specimens of Ecliinothuridse there are a few which I am unable to 

 refer satisfactorily to any one of the species here distinguished, and which are for the 

 present described under distinct names, the one as Phormosoma asterias and the other as 

 PTiormosoma rigidum, both these species differing from any of the species of Phormosoma 

 here described in having extremely narrow coronal plates more like the species of Asthen- 

 osoma allied to Asthenosoma grubii and Asthenosoma hystrix. 



* Phormosoma asterias n. sp. (PI. XII. " figs. 7-9). 



The only specimen collected measures 30 mm. in diameter, the whole abactiual 

 surface both in the ambulacra! and interambulacral areas is covered b}' primaries 

 arranged in somewhat irregular lines along the median lines of the plates (PL XII. " fig. 9) 

 with but few secondaries or miharies on the plates near the ambitus. On the actinal 

 surface, as in all young Phormosomas, there are from two to three primary tubercles 

 (PI. XII." fig. 8) with secondary tubercles and miliaries irregularly scattered over the 

 comparatively bare plates. The actinal membrane is as yet covered only hj the ten large 

 buccal plates with a couple of minute plates near the actinal edge of the test, the first 

 trace of the imbricating buccal j)lates of older stages. The course of the poriferous zone 

 is quite sporadic, and the Limitation of the ambulacra! and interambulacral areas ill- 

 defined near the actinostome. In addition to these features the abactinal system is 

 remarkable for the large size of the anal plates in so young a specimen, for the elongated 

 pear-shaped ocular plates and the large triangular genital plates, some of which show 

 traces of subdivision, as if the proximal extremities of the genital plates eventually 

 became cut off to form a part of the anal system. 



Colour of test light yeUo wish-red. 



Station 299. December 14, 1875. Lat. 33° 31' S., long. 74° 43' W. ; 2160 

 fathoms ; bottom temperature, 1'1° C. ; grey mud. 



*Phormosoma rigidum n. sp. (PI. XII.* figs. 1-4). 



The largest specimen collected measured only 40 mm. in diameter, and might readily pass 

 as the young of some species of Astropyga Avere it not for the characteristic Phormosoma 

 structure of the actinal surface (PI. XII. " fig. 3) and of the abactinal system. The 

 poriferous zone of this species is extremely narrow, more so than in corresponding stages 

 of other species of Phormoso7na (PL XII. ^ fig. 4), and it is still confined to the very outer 

 edge of the ambulacral zone. The coronal plates of the actinal surface in both areas 

 carry already two large primary tubercles (PI. XII." fig. 3), and the areolas show a slight 

 trace of difference of level with the surrounding test, so that the whole of the actinal 

 surface of the test appears thickly covered with large primary tubercles much as in Phor- 

 mosoma placenta. The coronal jjlates of the abactinal surface are narrow and elongated, 

 carrying, according to their size and position, from two to three primaries (PI. XII. " figs. 



