13 _ 



vided with irregular fcneslraled plates as in Ihe Echinidœ elc; on the side lying 

 towards the test these plates are enormously developed so as to form a strong 

 ridge which in dried specimens mostly adheres to the test, as a continuation 

 of the gillslit, and surpasses it in length ; in reality it is part of the gill itself. 



Auricula* and dental apparatus do not seem to give reliahle specific charac- 

 ters. Especially do I find the auriculæ very inconsistent in form and size. 



The irregular, usually triradiate, branched spicules in the tube feet have 

 been well figured by Stewart M, Perkier (Rech, sur les Pedicellaires p. 135. PI. 4. lb) 

 and Bell'-). They may be very few in number and present only in the upper part 

 of the tube foot, or rather numerous and lying in the whole length of the foot. 

 They are mostly arranged in three distinct series. The buccal tube feet, on the 

 other hand, are very richly provided with spicules in form of large, fenestrated 

 plates, a little curved according to the form of the foot, forming, as it were, a close 

 mail to the foot. In the abactinal tubefeet there is often found in the partition wall 

 a rather complicate calcareous network, formed of beams, which are widened to 

 small fenestrated plates at their outer ends (Pl.V. Fig. 12). Otherwise spicules are 

 not found in the abactinal tubefeet. 



The spines have been very carefully studied by Mackintosh"), to whose 

 descriptions and figures I must refer. He thinks specific differences exist in the 

 structure of the spines of the üioc/e;/)a-species, and he finds D. antillanim to be so 

 different from the other species in this respect that he is inclined to regard is as 

 a distinct form, in spite of the assertion of Agassiz that it is synonymous with Ü. saxa- 

 tile {setosuni). Though I completely agree with Mackintosh that D. untillarum is a 

 distinct species, I cannot agree with Iiim as to the structure of its spines. I find 

 them quite like those of D. saxatile, and upon the whole I cannot discover any reli- 

 able difference in the structure of the spines between any of the species of Diadema. 

 The figure (PI. XXXI. 8) given by Mackintosh as a transverse section of the 

 median region of the shaft of a primary spine of D. antillarum is probably taken 

 from an injured spine. In another treatise M Mackintosh mentions having found 

 dimorphic spines in D. mexicaimm „there being the usual long, tapering, verticil- 

 late spines with the normal Diadema structure which occur all over the test and a 

 limited number of fusiform ones placed on the actinal surface. These are about 

 half an inch in length, are longitudinally striated, not verticillate, as Diadema spines 

 mostly are. . . — It is also interesting that on the same specimen there occur a few 



M On tlie Spicula of the Regular Echinoidea. Transact. Linn. Soc. London. XX\'. 1865. p. 3(i.S. PI. 4ö. 

 Fig 17. a. 



') Note on the Spicules found in the ambulacra! tubes of the regular Echinoidea. Journ. R. niicrosc. 

 Soc. 2. ser. IL 1882. p. 2y7~99. PI. V. 



'} Researches on the structure of the spines of the Diadematidæ. Trans. R. Irish Acad. XXV. 1875. 

 p. 519—58. P!. XXXr— XXXlll. 



') Report on the Acantliology of the Desmosticha III. Further Observations on the Acanthology of 

 the Diadematidæ. Ibid. XX\1IL 1883. p. 259. 



