Report on the zoological collections made in the Indo-Pacific Ocean ...

;SPONGIIDA. 479a new genus by W. Weltner ('Beitr. zur Kenntniss d, Spongien,'Inaugural Dissertation, Freiburg-in-Ereisgau, 8vo, 1882, p. 30,pi. iii. figs. 29-41, 43), from Prof. Agassiz's dredgings in 1879off the Morro Light, Gulf of Mexico, in 250-500 fathoms. Inpoint of fact Weltner's species, which differs from T. dissimilis principallyin the possession of a fully developed triradiate "anchor,"occupies a position almost exactly intermediate between T. dissimilisand Stewart's species. I gather from Weltner's paper that he hasnot seen Stewart's description ; had he done so he would, I feelsure, have at any rate mentioned the close affinity of his speciesto that of Stewart, from which it differs chiefly by the elongateform of the flesh -stellate and by the suppression of the third lateralarm of the skeleton-spicule, a suppression already foreshadowed inStewart's species by the great reduction of two out of the threelateral ai'ms in some of these sjjiciiles (see fig. 75, I. c). I do notthink that TribracJiium can be upheld as distinct from Tetliyopsis ;the gradation of forms between T. columnifera and T. dissimilis,by which (1) the quadriradiate spicule of T. columnifera is reducedto a triradiate in Tribrachium, and to (a) a biradiate with abortedthird ray and {b) an acerate in T. dissimilis, together with the generalagreement between the minute spicules, the skeletal structure,and the general form of the sponge, appear to mark these threespecies out as belonging to a natural though highly plastic circleof forms com])arable to the Tetractiuellid genus Placina, Schulze,of which the species (P. inonolopha, dilopha, and trilopJia, Schulze)each include hi-, tri-, and quadriradiate forms of the fundamentalquadriradiate type ;they are comparable also to many genera ofthe Calcarea, where the fundamental (probably triradiate) type exhibitsgreat modifications, even within the limits of a single species.Besides possessing three complete arms and the large skeletonspicule,Tribrachium schmidti is distinguished from Tethi/opsis dissimilisby :—(2) the exterior being unmarked by horizontal ridges ;(3) the inferior length of the lateral arm of the triradiate spicule(4) the apparent absence of the long acerate spicule ; (5) the moregenerally elongate form of the flesh-spicule and the superior numberof its lateral whorls of tubercles.Weltner's comparison of the form of the minute flesh-spiculeswith the similarly dendritic skeleton-spicules of the IlhizomorineLithistids is invalidated by the fact that the two classes of spiculesare not homologous with each other, the flesh-spicules o^Trihrachiumbeing represented in the Lithistid series only by the minute bihamatesand other flesh-spicules of Corallistes &c.A striking analogy with the arrangement of the skeleton ofthe Lyssakiue Hexactinellida is aftbrded by the manner in whichthe arms and shafts of the large skeleton-spicules are employed inTetliyopsis (incl. Tribrachium) to form coherent rectangular meshes.Weltner's discovery is of great interest, apart from the ])eculiaritiesof the type described, in the fact that his species, though living inthe West Indies, is clearly intermediate between two types foundnear the confines of the Indo-Australian region.