660 MR. F. J. BELL ON THE ECHINOIDEA. [Julie 17, 



addition to this it is to be borne in mind that there is now valuable 

 evidence as to the fact that tubercles may, and do, undergo ab- 

 sorption ' ; so that we must not insist upon this character, where 

 others point to the contrary species as being in our hands. 



(5) Ocular plates. — I had hoped that these structures would 

 present some constancy of arrangement, which would be of assistance 

 in the discrimination of the species, inasmuch as in the great majority 

 of specimens of T. esculentus two only of the ocular plates reach to 

 the anal system (or, in other words, are not shut out from it by the 

 meeting of the edges of the genital plates ) : thus of six examples all 

 but one presented the arrangement just described, while the sixth 

 had four ocular plates directly adjacent to the anal system. T. angu- 

 losus presented no such constancy ; for out of nine examples there 

 were five that had two plates touching the anal system, while the 

 others had three plates occupying a similar position. No conclusions 

 can, therefore, be drawn from this character. 



Characteristic as is the arrangement of the pores in Tripneustes, 

 it is only of assistance in the definition of the genus ; when we come 

 to any close examination we find, as indeed we might expect from 

 what we know as to the mode of their development, that the arrange- 

 ment of the pairs of pores with relation to one another varies con- 

 siderably. I have noticed in large specimens of T. esculentus that 

 the inner row of pores is quite regular, while the outer row is, as 

 compared with it, irregular ; in the smallest specimens the two 

 flanking rows of pores exhibit very remarkable regularity, following 

 one another in quite straight lines. 



The specimens exhibiting a pentagonal aspect come in very large 

 quantities from the Red Sea; but there is in the Museum a specimen 

 from the Philippines in which this form of test is just as well marked 

 as in any Red-Sea specimen. 



There are some slight differences in the characters of the com- 

 ponent parts of the dentary apparatus (lantern of Aristotle), which 

 I will now proceed to indicate : — 



In T. angulosus the epiphysis is arched and its upper edge is 

 bevelled ; the tooth is connected with the alveolus by delicate, but 

 not very short, ascending and descending processes ; the rotulse are 

 short and broad ; and the radii end in two short processes. 



In T, depressus. the epiphyses are arched in very much the same 

 manner as in T. angulosus ; the inferior ascending processes are of 

 much the same character, but the superior processes are much 

 shorter ; the radius is broadened out at its free end, but there is only 

 a slight indentation at its extreme edge. 



In T. esculentus the epiphysis is less strongly arched, and its 

 upper edge is not so sharply bevelled ; the tooth is connected with 

 its alveolus by short pieces, which, above, are set nearly perpendicular 

 to it ; the inferior ones are only just seen through the triangular 

 space, or, in other words, extend hardly at all upwards ; the rotulse 

 are rather more delicate ; and the free end of the radius is distinctly 



1 Vide Rev. of the Echini p. 265, Arbacia pundulata. 



