Vol. 56.] FROM THE EOCENE OP VICTORIA. 51 



Petrostroma and the ' Griffin's Farm ' specimen. My warmest 

 thanks are due to Mr. Priest for placing at my disposal for description 

 all the specimens that he possessed. 



I propose, first, to describe in some detail the characters of the 

 4 Griffin's Farm ' specimen, which is placed as the type of a new 

 genus, Pleetroninia, and then to follow with a description of the other 

 forms. 



Plectroninia^ Halli, gen. et sp. nov. 

 (PI. Ill, figs. 1-83; PI. IV, figs. 1-11.) 



The type of the genus is turbinate in form, the base is blunted 

 and free, the lateral surface somewhat corrugated, and the summit 

 gently convex. It measures 16 mm. in height by 18 mm. in its 

 greatest width, which is just below the summit. It is now quite 

 free from matrix, except a small portion of the upper surface which 

 is covered with a greenish sandy clay containing subangular quartz- 

 grains. The summit is of a brownish-grey, while the lateral 

 surface is grey to silvery white ; the last-named tint, however, is 

 only shown where a too close application of a cleaning instrument 

 has removed the stained outer layer. The sides of the sponge were 

 originally covered by a spicular dermal layer, only in part preserved : 

 no traces of pores or canal-apertures could be detected in it. This 

 dermal layer did not apparently extend over the summit, which is 

 formed of the main spicular skeleton with open interspaces and the 

 apertures of excurrent canals. For a short distance below the 

 upper surface, the mesh-interspaces and canals are partly filled 

 with the fine granular matrix, in which are some foraminifera((xZo- 

 bigerina, Eotalia, Sjjiroloculina, etc.) and rarely diatoms showing 

 their minute structure. 



To study the interior structure, the sponge was sawn through 

 in a vertical direction from summit to base ; from one moiety thin 

 slices were cut off and mounted for the microscope in Canada 

 balsam, and fragments of the dermal layer were similarly mounted 

 in balsam. The interior mesh of the sponge, with the exception 

 of the stained portion near the summit, is of a dull white ; the 

 mesh-interspaces and the canals are quite free from any infilling, 

 save for wisps and irregular bands of loosely-arranged, delicate 

 spicules, apparently in the same position as during the life of the 

 sponge. The interior skeleton is of a firm, resistant character, not 

 unlike that of a recent Lithistid. 



Dermal Layer. 



The dermal layer or cortex, as already mentioned, is limited to 

 the lateral surface of the sponge ; it is thin, the outer surface mostly 

 smooth, but in places wrinkled and of a silvery white appearance. 

 This layer rests directly on the main skeletal mesh, except in the 

 basal portion of the sponge, where it passes into a subdermal layer of 



1 7r\fjKTpov, spur ; is, tVos-, fibre. 



e2 



