104 SILICEOUS SPOXGES. 



Pleueostoma bohemicum, Zittel. 

 1877. Pleurostoma bohemicum, Zitt. Studien, I Ab. p. 48; Neues Jahrbuch, p. 358. 

 Distribution. Upper Chalk : Plauen, Bohemia (coll. Zittel). 



Genus GUETTARDIA, Michelin, 1840-47. 



GuETTAEDiA STELLATA, Miclielin (pars). 



1840-47. Guettardia stellata, Michelin, Icon. Zooph. p. 121, t. 30. f. 3, 4, 6, 8-II, cet. excl. 

 1846. Guettardia Thio/ati, D'Areliiac, Mem. Soc. Geol. 2 ser. p. 197, t. 5. f. 15, and t. 8. 



f. 5, 6, 7. 

 1848. Brachiolites angularis, T. Smith, Ann. & Mag. Nat. Hist. vol. i. p. 357, f. O & P. 



The number of the radiating flanges in this well-known sponge varies in diff'erent 

 examples from 3 to 10, the parallel walls are from 1 to 2 mm. in thickness, and 

 about the same distance apart. The apertures on the outer lateral edges of the 

 flanges are ovate, and from 3 to 4 mm. in width. The base is usually rounded or 

 with an imperfect stem ; slender radical processes also appear to have originally 

 extended from the lateral edges of the flanges so as to serve in keeping the sponge 

 in an upright position. The central space appears to have been open above, and 

 there also appears to have been entirely free communication between the flanges and 

 the centre. There is great diversity in the size of difi"erent examples ; a small form 

 is 28 mm. in height by 30 mm. in width, and some of the large examples, judging 

 by the extension of a single flange, must have been 240 mm. in width by 100 mm. 

 in height. 



The walls carry thickly-set, minute, circular or oval canal-apertures, about '5 mm. 

 in width, disposed generally in quincunx. The spicules of the interior wall form an 

 irregular mesh with solid nodes. The dermal layer on the exterior surface, between 

 the canal-apertures, differs from the spicular mesh of the interior in a greater thick- 

 ness of the spicular arms, so that only minute circular pores remain between them. 



The most complete description of this species is that given by Toulmin Smith from 

 tlie examples in the English Chalk, which are either in the condition of rusty 

 peroxide of iron in the chalk itself, or preserved as reddish markings in flint. The 

 figure given by T. Smith, loc. cit. p. 358, fig. O, is, to a certain extent, diagrammatic, 

 for the original in the Museum collection is by no means so complete as represented. 



Michelin appears to me to have figured more than one species under G. stellata in 

 pi. 30, I. c. Thus in the flgures 2 and 5 the flanges grow upwards, and form 

 separate flattened branches, the same as in Pleurostoma, and only communicate with 

 the central cavity near their bases, and in this respect diff'er from the examples which 

 he has placed under figs. 3, 4, 6, 8, 9. For these latter forms I propose to retain 

 the name of G. stellata. D'Archiac has claimed one of these (fig. 6) as belonging to 

 his species G. Thiolati, but he might equally well have claimed the other examples 



