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Division II. CALCAREOUS SPONGES. 



Order CALCISPONGI^, Blaimille. 



Family PIIARETRONES, Zittel. 



The structure of this group of fossil sponges has lately been studied independently 

 by several authors, and as the result of their investigations has an important bearing 

 on the true character and affinities of the family, which have hitherto been the 

 subject of much discussion, it is desirable to make a preliminary reference to its 

 history, and to the views lately published respecting it. 



The family of the Pharetrones was constituted by Zittel to include a group of 

 sponges, very variable in outer form, but characterized by possessing a skeleton of 

 solid anastomosing calcareous fibres, so arranged as to form mesh-like walls of 

 varying thickness, either with or without distinct canals. The skeletal fibres, when 

 examined in thin transparent microscopic sections, were seen to be composed of 

 minute uniaxial or apparently three-rayed spicules ; but as the true form of these 

 minute spicules could very rarely be clearly seen in thin sections, some authorities 

 disputed their resemblance to the spicules of recent calcareous sponges, and regarded 

 them as siliceous spicules which had been changed to their present calcareous consti- 

 tution through fossilization. It was further maintained that, judging by the fragile 

 character of the spicules of existing calcareous sponges, it was altogether improbable 

 that similar forms would have been capable of preservation in the fossil state. 



In the course of my examination of the Museum collection, I met with some 

 examples of these fibrous calcareous sponges which supplied the evidence needed to 

 determine their true characters. The specimens were in such a peculiar condition 

 of preservation that I was enabled to detach the component spicules of the fibres 

 and to ascertain their forms with as much precision as if they had belonged to recent 

 sponges. The majority of these spicules were three- rayed forms of very minute 

 proportions, and in addition to these there was on the surface of the sponge a layer 

 of relatively large three- and four-rayed spicules, strikingly similar in form, size, and 

 in their relative arrangement to those of existing calcareous sponges. Further, in 

 thin microscopic sections of other forms, I detected, besides the common three- and 

 four-rayed spicules, some peculiar fork-shaped spicules, of a form quite unknown in 

 any siliceous sponge, either fossil or recent, but resembling in the greatest detail the 

 spicules of some recent calcareous sponges from the Australian coast. I submitted 

 microscopic fragments of these sponges to Prof. Zittel, who stated, in a short com- 



