58 DK. G. J. HINDE ON CALCISPOKGES [Feb. 1900, 



Cretaceous and Jurassic Pharetrones is yet more pronounced, for 

 the skeletal fibres of this sponge are more distinctly anastomosing, 

 and reticulate rather than radial in their arrangement, and until 

 examined under the microscope they could hardly, if at all, be 

 distinguished from those of many Pharetrones. Hitherto the 

 independent character of the spicules in the Pharetron fibres has 

 been generally accepted ; but now that the occurrence of organic 

 fusion has been proved in the case of these Lithonina, there is 

 room for suspecting that it may have also taken place in the fibres 

 of some of the Pharetrones. A renewed examination of my 

 microscopic sections of Pharetrones, in the light of the new 

 discoveries, leads me to think that there is very fair evidence that 

 fusion may have occurred in the fibres of such sponges as Sestro- 

 stomella rugosa, 1 Hinde, and Holcospongia floriceps 2 (Phill.), in 

 which the rays of large spicules extend longitudinally in the central 

 axis of the fibres and are closely enwrapped by smaller filiform 

 spicules. These larger axial spicules are four-rayed, one ray is 

 relatively short and pointed, the others are elongated, do not taper, 

 and apparently are truncate or slightly expanded at their ends. 

 These seem to be closely fitted to adjoining spicules, and resemble, in 

 their form and mode of union, the facial rays of the spicules in the 

 fibres of Plectroninia. In transverse sections of the fibres of 

 Sestrostomella rugosa the smaller spicules 3 appear also to be clasped 

 intimately to the larger, but whether this has been accompanied by 

 fusion or cementation is not definitely shown in the microscopical 

 sections at my disposal. The question whether the constituents of 

 the fibres in some of the Pharetrones are organically cemented 

 together or independent cannot be decided at present, but it 

 seems to me probable that eventually several genera in this family 

 will have to be placed with the Lithonina. 



Diagnosis of Plectroninia, gen. nov. 



Turbinate or rounded sponges, sessile or with a short pedicel, 

 the sides covered wholly or in part with a dermal layer of simple 

 pointed rods, styliform and pin-shaped forms, normal three- and 

 four-rayed and 'tuning-fork' spicules, free from each other, and 

 irregularly intermingled, except on the outer surface, which 

 is formed mainly of simple rods disposed parallel with each other. 

 At the base of the sponge, and also forming floors at different levels, 

 are thin layers consisting of four-rayed spicules of extremely 

 irregular and varied forms, which are attached together by the 

 clasping of the expanded termination of their facial rays to the 

 surfaces of adjacent spicules, but they are not apparently cemented 



1 Ann. & Mag. Nat. Hist. ser. 5, vol. x (1882) p. 198 & pi. x, fig. 6, pi. xii, 

 figs. 1-15 ; see also Catal. Foss. Spong. Brit. Mus. (1883) p. 188 & pi. xxxv, 

 figs. 4, 4 a-A d. 



2 See Palseont. Soc. Monogr. ' Brit. Foss. Spong.' pt. iii (1893) p. 226 & 

 pi. xvi, figs. 6-6 c, pi. xvii, fig. 2. 



3 Ann. & Mag. Nat. Hist. ser. 5, vol. x (1882) pi. xii, fig. 2. 



