60 DE. G. J. HINBE ON CALCJSPONOES [Feb. I9OO, 



sponges from the Eocene of Australia, evidently of the same generic 

 type as Bactronella pusilla, Hinde, now enables me to give the 

 following definition of the genus : — 



Sponges usually small and simple, varied in form, rod- or club- 

 shaped, rarely branching, conical, hemispherical, sessile, or with a 

 short pedicel, free or attached, or sometimes encrusting. The 

 outer surface is partly or entirely covered with a dermal layer ; and 

 there is also a basal layer of irregular four-rayed spicules, with the 

 apical ray free and the facial rays forming a mesh work by the 

 clasping together of the expanded ends of the rays, just as in 

 Plectroninia. The spicules of the main skeleton are likewise of 

 the same form, and firmly fixed together by the fusion of their 

 facial rays, as those in Plectroninia. They are disposed in such wise 

 that the free or partly free apical rays radiate upward and outward 

 to the surface of the sponge; and they are united transversely by 

 small spicules crossing the interspaces. The spicules bound minute 

 radial canals which open directly on the sides and summit of the 

 sponge. 



This genus is intermediate in character between Plectroninia and 

 Petrostroma. The spicules of the basal layer and of the main 

 skeleton resemble in form, and in their mode of union with each 

 other, those of the former genus, while in the radiate arrangement 

 of the skeleton, the small transverse spicules, and the radial canals, 

 there is great similarity to the latter. The apical rays of the 

 skeletal spicules are not so completely fused as in the balks of 

 Petrostroma Schulzei, and in some instances they are nearly free. 



Bactronella parvula, sp. nov. (PI. V.) 



Sponges diminutive, simple, pear-shaped or subcylindrical, with 

 rounded summits, encrusting other organisms, such as fragments of 

 polyzoa or detached spines of echinoderms (PI. V, figs. 1-4). They 

 range in size from 2 to 4 mm. in length, and from 1*2 to 1*5 mm. in 

 thickness. 



The outer surface of the sponge is dotted over with the rounded 

 apertures of the radial canals, which are from 0*1 to 0-12 mm. 

 in width ; their margins are formed by the three facial rays of 

 four-rayed spicules, and the fourth or apical ray projects radially 

 outward beyond the general surface of the sponge, thus giving it 

 a spinous appearance when viewed under the microscope (PI. V, 

 figs. 1-4, 16). The radial canals are very short tubes, corresponding 

 to the thin wall of the sponge. 



The basal layer in these sponges is developed in direct contact 

 with the foreign body over which they grow : in one sense it is 

 in the interior of the sponge, and is not exposed to view until a 

 section is made through the sponge. It consists of irregular four- 

 rayed spicules : the facial rays, short and truncate, being often armed 

 laterally with minute prickles, the apical rays pointed and free 

 (PL V, figs. 9, 12-14). These spicules are connected by inter- 

 clasping in the same manner as in Plectroninia, and apparently 



