PLATYCHONTA. 203 
cruciform spicules of moderate dimensions, about 1 mm. across; the spicules have 
no definite arrangement and their rays overlap each other irregularly. These 
spicules probably have a fifth ray which extends into the wall. In comparison 
with the specimens of 9. Lochense, Quenstedt, sp., from the Upper Jura of 
Germany, the present form has the oscules of the upper surface smaller and 
more closely arranged and the wall does not attain the same thickness. 
Distribution.—Inferior Oolite. Parkinsoni-zone in the Cliff-section at Burton 
Bradstock, Dorset. (Coll. Rev. G. F. Whidborne.) 
Sus-orper.—LITHISTID A. 
Family.—Raizomorina. 
Genus.—PuatycHonta, Zittel. 
1878. Studien iiber fossile Spongien, II]. Abband]. der k. bayer. Akademie der 
Wiss., Cl. ii, Bd. xiii, Abth. i, p. 114. 
Syn.—Spongites, Planispongia, Qwenstedt (in part); Amorphospongia, 
@ Orbigny (in part). 
Sponges with plate-like walls, either ear-shaped, undulating, or variously 
folded, less often cup- or platter-shaped. Both outer and inner surfaces of the 
wall with pores or ostia. Canal system scarcely distinguishable, in some 
instances apparently entirely limited to the labyrinthic interspaces formed by the 
skeleton, in others there are capillary-like tubes extending the entire length of 
the sponge and giving to it a fibrous, or radiate, appearance. The spicules 
consist of a main axis, usually curved; from the sides and ends of this spinous 
branches are given off. In some cases the lateral branches are so developed that 
the spicules can hardly be distinguished from the four-rayed Tetracladine spicules. 
The spicules are loosely interwoven so as to form an irregular meshwork. 
The typical species is Platychonia (Spongites) vagans, Quenstedt, sp. (‘ Der 
Jura,’ p. 679, pl. lxxxu, fig. 8), from the Upper, or White, Jura of Germany. As a 
rule, the sponges of this genus are so poorly preserved, that but little more than 
their form and mode of growth can be ascertained. 
