48 p. H. CAEPENTEK ON COMATUL^ FEOM 



distinct in some specimens than in others, both species exhibiting a 

 great amount of individual variation in this respect. In the Heard- 

 Island specimen represented here, both the radial and the inter- 

 radial ribs are exceptionally distinct. 



The presence of these ribs is very characteristic of the two polar 

 Antedons. It does not appear to depend upon the depth of the 

 centrodorsal cavity ; for there are no ribs even in the deepest centro- 

 dorsal pieces of the northern A. celtica or of A. phalangium (Medi- 

 terranean), though these centrodorsals may be externally higher 

 and internally deeper than those of small examples of A. Eschrichtii. 

 The same is the case with regard to the deep centrodorsals of some 

 new Antedons from the South Pacific. Only one of these, from 

 Station 170 (630 fathoms), just north of the Kermadec Islands, 

 shows any trace of ribs. There are only ten, five radial and five 

 interradial, and they are so faintly marked that they would readily 

 be overlooked by any one whose attention was not specially directed 

 to them. 



Now in Goldfuss's description of A, paradoxa (p. 159) the fol- 

 lowing passage occurs : — 



" The mouth is furnished with five blunt processes, only one of 

 which is preserved in the specimen figured ; so that the points of 

 attachment of the rest are visible." According to Schliiter *, how- 

 ever, "the tooth-like process described by Goldfuss in the mouth is 

 nothing but a fragment of the calcareous matrix which has by 

 chance adhered to the specimen at this point." It will be under- 

 stood that the term *' mouth " is here used to mean the central 

 opening on the ventral surface of the centrodorsal. 



Schliiter has examined the original specimen figured by Goldfuss, 

 and therefore speaks with authority ; but I cannot help suspecting 

 that the interradial process in question was of the same nature as 

 the larger ribs of A. Eschriclitii and of its southern representative 

 (PI. Y. fig. 3). In the English specimen which I am describing 

 these interradial ribs are rather prominent; they stand out like 

 buttresses, projecting into the central cavity from the inner ends of 

 the basal grooves which they support; while there are traces of 

 smaller intervening ribs on the end walls of the radial extensions of 

 the central cavity — faint ones, it is true, in four cases, but tolerably 

 distinct in the fifth. It is the presence of these ribs which leads me 

 to think (as pointed out above) that the radial extensions of the 

 central cavity were continuous with it beneath bony bridges, and 

 not cut off from it as in ^. Betzii ; so that they are not precisely 

 equivalent to the radial pits of A, rosacea or A. rotunda (PI. V. 

 fig. 5 a), which are merely excavations in the projecting lip of the 

 cavity or in the upper surface of its walls. 



Diameter of larger specimen, 13 millims. ; of smaller, 6 millims. 

 Heights, 7 millims. and 3 millims. respectively. 



Locality. Upper Chalk, Dover. Coll. Museum of Practical 

 Geology. 



* 0^. cit p. 43. 



