Mr. W. J. Sollas on the Genus Catagma. 363 



multiradiate spicules to the Calcispongia, but also the genus 

 Pharetrospongia , in which none but uniaxial spicules exist. 

 Now, whatever uncertainty exists about the Faringdon and 

 Upware sponges (and I admit a great deal), there is, I am 

 confident, none here. The proofs as to tlie nature of Phare- 

 trospongia are, I believe, perfect ; and should the Faringdon 

 sponges eventually turn out to be genuine Calcisponges, I do 

 not see how that can for a moment affect the position of 

 Pharetrospongia. 



Its structure differs in no important respect, except in the 

 absence of flesh-spicules, which fossilization would inevitably 

 destroy, from that of a recent siliceous sponge {Pharetronema^ 

 Sollas) which I have now before me awaiting description. 

 Both are exactly similar in the size and shape of their spicular 

 elements, in the arrangement of these spicules in a fibrous 

 manner, in the thickness and character of the sponge-wall, in 

 the form of the fibrous skeleton, and, finally, in the absence of 

 obvious pores, oscules, and excretory canals. Were the two 

 to be found together in the fossil state it would be difficult to 

 distinguisli one from the other, except by a very slight differ- 

 ence in external form. While this exact agreement exists 

 between Pharetrospongia and a modern siliceous fibrous sponge 

 (and similar sponges are amongst the commonest of our exist- 

 ing seas), there is no resemblance, but the most absolute differ- 

 ence, between it and any known form of the Calcispongia. 

 Furthermore, we can fortunately, in this instance, adduce the 

 mineral state of the sponge in support of the morpliological 

 argument, since the silica removed from its calcitized spicules 

 has been deposited in the infilling material of its meshwoi-k, 

 and many, a great many, of its spicules retain their original 

 siliceous composition. 



So seldom does a calcareous organism in the Cambridge 

 Greensand become silicified, and so constantly does Pharetro- 

 spongia possess both siliceous spicules and a silicified matrix, 

 that one cannot regard the ])resence of the silica as due to its 

 subsequent introduction. The manner of its occurrence and 

 the perfect form of the spicules, which are still siliceous, leave 

 no doubt in my mind, independently of the morphological 

 structure of the sponge, that the original composition of these 

 Cambridge sponges was siliceous and not calcareous. 



With regard to Pharetrospongia, then, we have to take 

 nothing upon trust, to make no assumptions, to imagine 

 nothing ; the evidence is as perfect in its separate links, and 

 as complete in the union of these links, as it is in the nature 

 of palseontological evidence to be. 



Bristol Museum, 

 Sept. 30, 1878. 



