Vol. 56.] FROM THE EOCENE OF VICTORIA. 57 



The Relations of Plectroninia to other Calcisponges. 



In the principal characteristic feature, the organic fusion or 

 welding together of the spicules of the skeletal mesh, this genus 

 bears a distinct relationship to the recent Petrostroma Schulzei, 

 Doderl. the only calcisponge, recent or fossil, in which up to the 

 present a definite fusion of the skeletal spicules has been shown to 

 take place, and there can be no doubt that it will come under the 

 group of Lithonina, proposed by Rauff to include sponges with this 

 character. At the same time there are considerable differences 

 between Petrostroma and Plectroninia. To take first the spicules of the 

 skeletal mesh (stiitzske let), these in Petrostroma have their facial 

 rays 1 (cladiske) usually curved and tapering to a blunt extremity, 

 whereas in Plectroninia they are truncate with expanded ends 

 (PI. IY, figs. 7 & 8), and it is only in very exceptional instances that 

 a facial ray resembles those of the recent sponge. With this 

 difference in the form of the facial rays there is also a corresponding 

 variation in their mode of union, which in Petrostroma appears 

 to be effected by lateral fusion wherever they are in contact 

 with adjacent rays, instead of by a close fitting of the terminal 

 facets of the rays and subsequent fusion. Again in Petrostroma 

 the skeletal fibres have the form of relatively thick balks, 2 

 connected transversely by minute spinous spicules, which radiate 

 to the surface of the sponge, whereas in our sponge the fibres 

 are regularly reticulate, and not pronouncedly radial in direction. 

 Moreover, the canal-system in Plectroninia differs markedly from 

 that of Petrostroma, and the characters of the dermal and basal 

 layers in the two genera are also unlike. 



There are two other genera of fossil calcisponges known to me in 

 which the spicules are fused together like those of Plectroninia and 

 Petrostroma : one is Bactronella, 3 Hinde, which will be referred to 

 later on (p. 59), and the other Porospheera* Steinmann, placed by 

 this author as a Hydrocoralline allied to Hillepora. The evidence 

 of the affinity of this genus to the Lithonina I hope to publish 

 shortly, and will now only remark that it is sufficiently distinct from 

 Plectroninia to allow of the validity of this latter. 



Prof. Doderlein 5 has commented on the striking general resem- 

 blance between the skeletal fibres of Petrostroma and those of genuine 

 Pharetron calcisponges, and also on the occurrence, in both groups 

 of sponges, of the very specialized ' tuning-fork ' spicules ; but, on 

 the ground of the independent character of the spicules in the fibres 

 of the Pharetrones as compared with their fused condition in 

 Petrostroma, he concludes that these two kinds of sponges have 

 nothing to do with each other. The likeness of Plectroninia to 



1 Zool. Jahrb. vol. x (1898) pi. ii, figs. 31-34. 



2 Ibid. pi. iv, fig. B. 



3 Cat. Foss. Spong. Brit. Mus. (1883) p. 205. 



4 Palseontographica, vol. xxv (1878) p. 120. 



5 Zool. Jahrb. vol. x (1898) p. 28. 



