DIAPLECTIA.— ELASMOSTOMA. 193 



Genus DIAPLECTIA, Hinde, n. g. 



Sponges growing in cup-, f;in-, or platter-shaped expansions. The walls thioughout 

 consist of the fibrous mesh. No canals are present ; the surface on both sides of the 

 wall exhibits only irregular interspaces between the fibres. The spicular structure 

 of the fibres resembles that of Sestrostomella. 



I propose this genus to include Spongia helveUoides, Lamx., and some other allied 

 forms which, from their similar mode of growth, have been included by Zittel in the 

 genus Pharetrospongia, Sollas ; but they possess an entirely different spicular structure 

 from that of the type of this latter genus. 



DiAPLECTiA AUEicuLA, Hifide, n. sp. (Plate XXXVI. figs. 4, 4 a.) 



Sponges forming fan- or ear-shaped expansions. The type specimen is 30 mm. in 

 height and 50 in width. The wall varies from 5 to 10 mm. in thickness ; the margins 

 are obtusely rounded. The fibres are from "2 to -4 mm. in thickness; they are made 

 up of relatively large thi"ee- and four-rayed spicules in the axis, and smaller sinuous 

 spicules bordering the fibre. Minute pitch-fork spicules, like those of Sestrostomella, 

 are also present. 



Distribution. Inferior Oolite (Pea Grit) : near Cheltenham. Middle Jura : Couche 

 a polypiers ; Langrune (coll. Tesson). 



DIAPLECTIA HELVELLOIDES, Lamx. sp. (Plate XXXVI. fig. 5.) 



1821. Spongia helveUoides, Lamx. Exp. method, p. 87, t. 84. f. 1-3. 

 1854. Spongia helveUoides, Morris, Cat. Brit. Foss. p. 30. 

 1878. Pharetrospongia helveUoides, Zitt. Studien, 111 At. p. 46. 



Cup-shaped sponges supported on a small cylindrical stem. The walls are from 

 3 to 5 mm. in thickness. The fibres of the outer or under surface of the cup have a 

 generally vertical disposition; they vary from '14 to -4 mm. in thickness. The 

 spicular structure resembles that of the preceding species. 



This may be distinguished from the preceding species by its form, thinner walls, 

 and the vertical disposition of the fibres of the outer surface. 



Distrihution. Middle Jura: Couche a polypiers — Langrune, lianville, near Caen 

 [coll. Tesson). Great Oolite : Hampton CliflF, Bath [Morris). 



Genus ELASMOSTOMA, From. 1859. 



In the typical species of this genus E. acutimargo, F. A. Roem. {=E.frondescens, 

 From.), the spicular structure of the fibres is of the Sestrostomella type, consisting of 

 central, relatively large, three- and four-rayed spicules, surrounded by smaller sinuous 

 irregular spicules, also probably tliree-rayed. A similar spicular structure is shown 

 in E. Norynanianum, D'Orbig., and E. consohrinum, D'Orbig. sp., as well as in some 



2c 



