392 Mv. H. J. ('alter on the " Tuhulations Sableuses.'" 



organic substance ; for if it had been hard and calcareous like 

 that of coral, it could not have been replaced by the quartz 

 sand of the deposit. Of course, also, the variety of specimens 

 from which the above characters have been taken is limited, 

 and therefore open to alteration by those who may have 

 the opportunity of observing an unlimited number on the 

 spot. 



Lastly, as regards the presence of sponge-spicules being 

 indicative of that of distinct species of the Spongida. Had 

 this been the case, in the first instance, viz. where the spicules 

 had heen formed hy the sponge itself] they would have been all 

 of one kind, more or less entire and regularly arranged. On 

 the contrary, they are of fifty or more different kinds and 

 forms belonging to as many difierent species of sponges, 

 nearly all more or less fragmentary and thrown together 

 most confusedly. Still this heterogeneous assemblage, in- 

 cluding all sorts of other minute organisms, might be exactly 

 the case with the arenaceous sponges (ex. gr. Dysidea) , which 

 do not form their own spicules, but, for the sake of obtaining 

 solid material for their skeletons, take in every thing of 

 the kind that impinges upon their surface. But this is 

 done by the Dysidea in a massive, amorphous, lobed form, 

 while the tuberculous layer of the " tubulation sableuse," as 

 before stated, has, where most regular, a defined pattern on its 

 surface, is chiefly composed of pure quartz sand like that of 

 the cylinder and surrounding deposit, and is only coated by a 

 layer of the spiculiferous heterogeneous material mentioned. 

 Like, therefore, as the cylinder and its tuberculous crust in 

 Broeckia hruxellensis is to a fossil sponge of this form, the 

 detail is totally opposed to such an inference. 



Considered apait, however, the great number of these sj)i- 

 cules thus brought together, their variety, state of preservation, 

 and the easy way in which they are extricated from the friable 

 material Avith which they are combined, render them a 

 striking and valuable records of the orders and species of 

 the Spongida which existed at that epoch and in this loca- 

 lity. 



EXPLANATION OF PLATE XVIIL 



N.B. All these representations are of tlie natural size, and in their 

 outline almost facsimiles of the objects themselves. 



Fig. 1. Broeckia hnixellensis, nov. gen. et spec, divested of the concre- 



tionars' crust, a a, tuberculous layer ; 6, central cylinder ; c, 



posterior or pointed extremity. 

 Fig. 2. The same : partly covered by the concretionary crust, which has 



been split open to show the interior, a a, concretionary cruet ; 



bhhh, mould of the tuberculous layer left in the concretionary 



