SPONGIIDA. 479 



a new genus by W. Weltner ('Beitr. zur Kenntniss d. Spongien,' 

 Inaugural Dissertation, Freiburg-in-Breisgau, 8vo, 1882, p. 30, 

 pi. iii. figs. 29-41, 43), from Prof. Agassiz's dredgings in 1879 

 off the Morro Light, Gulf of Mexico, in •250-500 fathoms. In 

 point of fact Weltner's species, which differs from T. dissimilis prin- 

 cipally in the possession of a fully developed triradiate " anchor," 

 occupies a position almost exactly intermediate between T. dissimilis 

 and Stewart's species. I gather from Weltner's paper that he has 

 not seen Stewart's description ; had he done so he would, I feel 

 sure, have at any rate mentioned the close affinity of his species 

 to that of Stewart, from which it differs chiefly by the elongate 

 form of the flesh-stellate and by the suppression of the third lateral 

 arm of the skeleton-spicule, a Suppression already foreshadowed in 

 Stewart's species by the great reduction of two out of the three 

 lateral arms in some of these spicules (see flg. 75, I. c). I do not 

 think that Tribrachium can be upheld as distinct from Tethyopsis ; 

 the gradation of forms between T. colwmnifera and T. dissimilis, 

 by which (1) the qiiadriradiate spicule of T. colwmnifera is reduced 

 to a triradiate in Tribrachium, and to (a) a biradiate with aborted 

 third ray and (6) an aoerate in T. dissimilis, together with the gene- 

 ral agreement between the minute spicules, the skeletal structure, 

 and the general form of the sponge, appear to mark these three 

 species out as belonging to a natural though highly plastic circle 

 of forms comparable to the Tetractinellid genus Placina, Schulze, 

 of which the species (P. monoloplia, dilopJia, and trilopha, Schulze) 

 each include bi-, tri-, and quadriradiate forms of the fundamental 

 quadriradiate type ; they are comparable also to many genera of 

 the Calcarea, where the fundamental (probably triradiate) type ex- 

 hibits great modifications, even within the limits of a siiigJe species. 

 Besides possessing three complete arms and the large skeleton- 

 spicule, Tribrachium schmidti is distinguished from Tethyopsis dis- 

 similis by : — (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 more 

 generally elongate form of the flesh^spicule and the superior number 

 of its lateral whorls of tubercles. 



Weltner's comparison of the form of the minute flesh-spicules 

 with the similarly dendritic skeleton-spicules of the Ehizomorine 

 Lithistids is invalidated by the fact that the two classes of spicules 

 are not homologous with each other, the flesh-spicules otTribraehium 

 being represented in the Lithistid series only by the minute ■ biha- 

 m9.tes and other flesh-spicules of Oorallistes &o. 



A striking analogy with the arrangement of the skeleton of 

 the Lyssakine Hexactinellida is afforded by the manner in which 

 the arms and shafts of the large skeleton-spicules are employed in 

 Tethyopsis (incl. Tribrachium) to form coherent rectangular meshes. 



"Weltner's discovery is of great interest, apart from the peculiarities 

 of the type described, in the fact that his species, though living in 

 the West Indies, is clearly intermediate between two types found 

 near the confines of the Indo-Australian region. 



