PLATYCHONIA AFFINIS. 205 
together, and they are now only indicated in thin sections by patches of calcite 
with irregular outlines (Pl. XII, fig. 26). This will be better understood by 
comparison with the figures of a portion of the spicular mesh of a closely allied 
species, Platychonia vagans, Quenstedt, sp., in which the spicules retain their 
siliceous structure (Pl. XII, fig. 3). In its mode of growth and in the thickness 
of the walls this species has a considerable resemblance to P. vagans, but owing 
to its condition of preservation a close comparison is impracticable. 
Distribution.—Inferior Oolite. Parkinsoni-zone in the Cliff-section at Burton 
Bradstock, Dorset. (Coll. Rev. G. F. Whidborne ; G. J. Hinde.) 
12. Piatycnonia tenuis, Hinde, sp. nov. Plate XII, figs. 6, 6a. 
Sponges consisting of thin plate-like walls, either ear-shaped or convolute ; 
the walls are from 3°5 mm. to’5 mm. in thickness ; the surfaces are fairly even though 
with occasional depressions. The outer surface has small circular or irregular 
ostia from *2 mm. to’*5 mm. in width. The spicular mesh is of the same character as 
in P. elegans, but the component spicules are apparently smaller. The specimens 
range from 35 mm. to 68 mm. in width. 
This species differs from P. elegans in its thinner walls, more even surfaces, 
and more delicate spicules. 
Distribution.—Inferior Oolite. Parkinsoni-zone in the Cliff-section at Burton 
Bradstock, and at Shipton Gorge, Dorset. (Coll. G. J. Hinde ; H. A. Walford.) 
13. Puarycnonta aFFINIs, Hinde, sp. nov. Plate XII, figs. 5, 5 a. 
The only specimen referred to this species is platter-shaped, with an oval 
outline, 90 mm. across; it has a short, inversely-conical base, whilst the margins 
are rounded and in places thickened and recurved. The walls range from 5 to 
10 mm. in thickness. The under or outer surface exhibits small rounded pores, or 
ostia, about -4mm. in width, bounded by the skeletal fibres ; in places on the 
upper surface there are traces of horizontal canals from *5 mm. to *7 mm. in width. 
The skeletal mesh consists of fibres of loosely-arranged spicules which are now 
entirely replaced by calcite. In form and mode of growth and in the more 
distinctly fibrous character of the spicular mesh this species is distinguished from 
the preceding. 
Distribution.—Inferior Oolite. Parkinsoni-zone at Burton Bradstock. (Coll. 
G. J. Hinde.) 
DD 
