Vol. 56.] FROM THE EOCENE OF VICTORIA. 63 



round the cloaca. Small specimens are not more than 5 mm. in 

 diameter ; the largest form examined is 13*5 mm. in height, by 15*5 

 mm. in thickness, and the wall is 3 mm. thick. Not infrequently 

 the sponges are traversed by smooth curved tubes, sometimes with 

 definite calcareous walls, belonging to some organism which has 

 either bored into the sponge or has been overgrown by it. 



There is a well-marked system of excurrent canals which follow 

 the contour of the sponge, and at the surface these are indicated 

 by open, slightly curved furrows, which extend up the sides and 

 over the summit to the cloaca, into which they open. The outer 

 surface of the sponge, as well as the cloaca, is penetrated by 

 numerous oval or rounded canal-apertures about 0*35 mm. in width. 

 As viewed in section the canals are from 0*11 to 0-35 mm. wide. 



The skeleton consists of continuous, anastomosing, narrow fibres, 

 from 0*06 to 0-09 mm. in width, disposed so as to form a mesh with 

 small interspaces (PI. IY, fig. 27). The spicules of the fibres have short 

 rays apparently with truncate ends, which, so far as can be seen, are in 

 close contact and perhaps fused together ; but on this point no con- 

 clusive evidence can be obtained, for the structure is very indistinctly 

 preserved, and the fibres only show the spicular rays either singly 

 or two or more ranged side by side (PI. IV, figs. 28 & 29). The rays 

 are about 0'16 mm. in length by O04 mm. in thickness. The wall of 

 the cloaca is well-marked and relatively thick ; its structure differs 

 from that of the skeletal mesh, but neither the individual spicules 

 nor their mode of union can now be distinguished. 



In the regular anastomosis of the skeletal mesh and in the 

 character of the canal-system, these sponges do not differ from 

 normal Pharetrones, and whether they should be placed under this 

 group or with the Lithonina depends on the mode of union of the 

 spicules of the fibres, a feature which is not shown with certainty 

 in any of the specimens that have as yet been examined ; but the 

 character of the spicular rays indicates that they may have been 

 firmly cemented together in the same way as in the other sponges 

 from the same horizon described in the foregoing pages. The regular 

 disposition of the mesh, the non-radial arrangement of the fibres or 

 of the spicular rays, and the canal-system, sufficiently differentiate 

 this form from Plectrordnia and BactroneUa. 



Distribution. — Not uncommon in the Eocene limestone at 

 Flinders (Victoria), associated with BactroneUa austral^, Hinde. 

 Collected by Mr. T. S. Hall, M.A. 



EXPLANATION OF PLATES III-V. 



Plate III. 



Plectroninia Halli, gen. et sp. nov. 



Pig. 1. The type-specimen, natural size. From Eocene clays at Griffin's Farm, 

 Moorabool Eiver (Victoria). Collected by Mr. T. S. Hall, MA. 

 1 a. The same, showing a vertical median section. X 2. 

 Figs. 2 & 3. Two small specimens, natural size. From Eocene beds at Flinders 

 (Victoria). Collected by Mr. T. S. Hall. 



