1879.] MR. F. J. BELL ON THE ECHINOIDEA. 659 



diameter. Taking note of this lapse, we will first consider those 

 statements regarding the species in which the results to be gained 

 from an examination of the British-Museum specimens are not in 

 complete accordance with the deductions of Prof. Agassiz : — 



(1) Comparative breadth of the poriferous zone. — Prof. Agassiz 

 states (p. 501, s. v. H. variegata) that "the poriferous zone is also 

 much narrower." As I found that my own measurements reversed the 

 relation, and led me to the conclusion that it was in T. esculentus 

 that the zones were narrower, I have been at the trouble of re- 

 ducing the figures in the ' Revision ' to a percentage value ; and I 

 find them to be 



For " H. esculenta" 10, 8-3, 8-9, 77. 

 For " H. variegata," 9*0, 8, 7. 



Pruned of its epithet " much " the statement of Agassiz is supported 

 by his data. The British-Museum specimens, which I have measured, 

 do not exhibit so great a range of variation in the width of the pori- 

 ferous zone, as may be seen from the appended list : — 



T. angulosus, 8, 8'2, 8-2, 8-6, 89, 9, 9. 

 T. esculentus, 7*8, 8, 8*4, 8'5. 



These observations indicate that the poriferous zones are rather 

 narrower in T. esculentus than in T. angulosus; but they really run 

 so close that it seems to me that it is impossible to find in this 

 character any constant or valuable point by which the two species 

 may be distinguished. 



(2) Characters of the actinostome. — The relative size of the actino- 

 stome in Tripneustes angulosus as compared with that of T. esculentus 

 is one of the few points of difference to which it is, as a rule, easy 

 to point. In connexion with it there is another character, which it is 

 perhaps safest to speak of as a tendency : in T. angulosus the ac- 

 tinal surface is, as a rule, perfectly flat, and the actinostome is flush 

 with it, whereas in T. esculentus that same surface is ordinarily a 

 little swollen, and the actinostome is placed in a shallow concavity. 



(3) The anal system. — It is interesting to compare the data 

 afforded by the specimens of T. esculentus in the British Museum 

 with those given by Prof. Agassiz. These latter are respectively 7"9, 

 8"1, 9"1, 9 - 2 ; those which are now given in the Table appended are 

 66, 7, 7'4, and 8 ; and they are to the point as leading us to insist 

 a little more strongly on the comparatively smaller size of the anal 

 system in T. esculentus than we should be justified in doing from a 

 knowledge of Prof. Agassiz's measurements alone. 



(4) Difference in the size and number of the tubercles. — This 

 appears to be a good character ; but we must insist upon the fact that 

 specimens of T. esculentus will be met with which have the median 

 primary tubercles of the abactinal surface largely absorbed, while, 

 on the other hand, there is in the Museum a young specimen which 

 in the characters of its actinal and abactinal systems approximates 

 to T. angidosus, but in which we find considerably well developed 

 tubercles in the median spaces of the interambulacral areas. In 



