POROCIDARTS COBOSI. 31 



when there are not more than two or three pairs, have ah-eady become, 

 in great part, joined laterally. On the buccal plates a single row of 

 secondary tubercles, separated by miliaries irregularly arranged, extends 

 along their actinal edge, leaving the rest of the plate bare (PI. 11, figs. 7, 2). 

 In young specimens two or three miliaries only are found on each plate 

 (PI. 12, figs. ^, 3). 



The separation of the actinal ambulacral plates and their migration 

 upon the actinal system is well seen in PI. 12, figs. ^, 5. In one case 

 (PI. 12, fig. 5), the ambulcral plate has become separated as a whole ; in 

 the other (PI. 12, fig. 4) the ambulacral plate has been torn off in such 

 a way as to carry with it a part of the next ambulacral plate as -well as a 

 part of the adjoining interambulacral plate. In Plate 12, figs. G and 7, 

 is seen the actinal edge of the odd interambulacrum, showing the extent 

 to which the actinal plates have been resorbed, and indicating (PI. 12, 

 fig. 7) the minute calcareous plates which have filled the space left between 

 the distal ambulacral buccal plates. 



In Plate 11, figs. 3, we have the actinal plates of the odd ambulacrum 

 and the adjoining interambulacra seen from the actinal side; Plate 12, 

 fig. ^, shows the same ambulacrum seen from the interior to show the 

 auricles and the remarkable processes extending from the abactinal side of 

 the ambulacral buccal plates. 



Thomson ^ has figured for Porocidaris piirpurata the peculiar curved spines 

 into which the buccal ambulacral plates rise on each side of the median 

 line as seen from the interior of the test. I do not know similar plates 

 in other Cidarida3. 



Thomson also speaks^ of the two-valved pedicellarioe of Porocidaris 

 as unique among the Echinoideans. They occur in ^Clypeastroids and 

 Spatangoids. 



Station, 3404, off Chatham Id. (Galapagos), 385 fathoms, Lat. V 3' 

 S. ; Long. 89° 28' W. Bottom temperature, 43°.2. Rocky. 



1 Thomson, Wyville, Echinoidea of the "Porcupiue" Deep Sea Dredging Expedition. Trans. 

 R. S. London. Vol. 164, Pt. 2, p. 728, fig. 2. 



2 Loc. cit., p. 727. 



