116 Thos. Roberts — Cretaceous Echinoids. 



so as to exhibit a microscopic botryoidal appearance, the globules or 

 discs ranging from "01 to "03 mm. in diameter. This globular form 

 of opal silica is similar to that which occurs in many of the sponge- 

 beds of the Upper Greensand ' in this country and there can hardly 

 be any doubt that in this Australian Chert it is due, as in the Chert 

 of this country, to the solution and redeposition of the organic 

 silica of the sponge-spicules. 



The spicules in the Australian rock are not sufficiently well pre- 

 served to indicate with certainty the sponges to which they belonged ; 

 the only forms recognizable are portions of long slender rod-like or 

 acerate spicules from '5 to 1-4 mm. in length, and "02 to '06 in thick- 

 ness, and calthrop or four-rayed spicules similar to those in the 

 genus Pacfiastrella, 0. Sch, It seems probable that they are chiefly 

 of Tetractinellid sponges. 



The principal importance of this Chert is the evidence it affords 

 of the origin of this rock in Australia from organic sources ; in view 

 of the fact that Chert in Australia has been definitely attributed to 

 chemical deposition ^ merely, the evidence to the contrary is the 

 more valuable. 



The specimens were collected by Mr. Brown at Yorke's Peninsula, 

 near Adelaide, and the beds are believed to be of Tertiary age, 

 though, so far as I am aware, their particular horizon has not been 

 ascertained. 



v. — woodwakdian mttsbum notes. 



On Two Abnormal Cretaceous Echinoids. 



By T. Egberts, M.A., F.G.S. 



IN 1887 a quadri-radiate Echinoconus (Galerites) suhrotundus from 

 the Lower Chalk of Shudy Camps, Cambridgeshire, was 

 presented to the Woodwardian Museum by J. Carter, Esq., F.G.S. 

 The specimen is in a good state of preservation, but it has been 

 subjected to a slight amount of crushing, and this has caused (1) the 

 apex of the test to be pushed backwards to a very small extent, and 

 (2) an abrasion of the antero-lateral border. It has the subconoidal 

 form of the normal examples of this species, but is subquadrate in 

 section, and this is due to the fact that only four ambulacral and 

 four interambulacral areas are present, the fifth area of both kinds 

 being completely wanting. 



Both the ambulacral and interambulacral areas are relatively 

 wider than those in a normal specimen of the same size, and the 

 space of the areas that are wanting appears to be shared propor- 

 tionately among the eight areas that are present. 



The apical disc is compact and roughly rectangular ; the outline 

 of the individual plates is not clear, but the disc is seen to be com- 

 posed of four perforated basals (genitals) and four perforated radial s 

 (oculars). Even the basal plate corresponding to the posterior 

 unpaired interambulacral area is perforated, which in normal 

 specimens is imperforate. 



1 Phil. Trans, vol. ckxv. pt. ii. 1885, p. 428. 



2 Hull and Hardman, Proc. Eoyal Soc. vol. xlii. (1887), pp. 304-310. 



