480 MR. S. O. RLDLET ON THE GENUS DIRRHOPALUM. 



should perhaps, looking to the peculiarity and antiquity of its 

 characteristic spicule (4), be made the type of a distinct genus. 

 I shall, for the present, allude to forms which resemble it as 

 belonging to the u clopetarium section " of the genus Dirrhopaltm. 



3. DlRRHOPALUM PLENUM. 



Plocamia plena, Sollas, Ann. fy Mag. N. H. (5) iv. p. 44, pis. vi. & vii. 



A member of the typical section of the genus, i. e. of that part 

 which is represented by D. gymnazon, for it has a smooth- 

 shafted dumbbell spicule coexisting with equianchorate and tri- 

 curvate flesh-spicules. The short-spined acuate (plate vi. fig. 5, 

 <fcc), with the coarse and backwardly-directed spines of its shaft 

 and the somewhat tubercular spines of its base, may perhaps 

 represent the pegtop-like form of D. clopetarium, and thus the 

 species may form one link in the chain, if it ever existed, between 

 that species and D. gymnazon. This may well be, for if D. clo- 

 petarium is ultimately found, like D. gymnazon, to have the flesh- 

 spicules, the only important points then separating it from D. plenum 

 would be the tuberculation of the shaft of the dumbbell spicule, 

 the tuberculate character of the short-spined acerate, and possibly 

 (and, if so, most important of all) the non-echinating position of 

 this spicule, which is distinctly an echinating form in D. plenum. 



The arrangement of the skeleton of D. plenum is also typically 

 Dirrhopaline, showing a vertical or primary fibre echinated by an 

 acuate and subspinulate spicule, and a horizontal or secondary 

 fibre or tract containing the dumbbell form. The yellow colour 

 ascribed to the sarcode, and the firm consistency of the skeleton, 

 appear to me to indicate that there is a decided admixture of a 

 ceratinous element, or of some analogous substance, in it, in 

 spite of Mr. Sollas' s conclusions derived from facts of some 

 importance. "Whatever, however, may be the case with this 

 species, it certainly seems to occur in an undoubted Dirrho- 

 palum, viz. D. manaarense, Carter, which I have examined, where 

 its prominence is the most striking point about the fibre of the 

 stem, when freshly mounted in balsam or when treated with strong 

 alcohol. I am inclined to think that some forms of ceratinous 

 material have a refractive index so near that of Canada balsam 

 as to be hardly distinguishable when mounted in that medium. 

 In opposition to this view, however, Sollas's experiments with 

 glycerine jelly still remain. The firmness of union of the various 

 spicules in this Echinonematous genus seems to demand some 



