PHAEETEONES. 159 



similar to that of the recent Leucones, and that its present fibrous character is 

 merely the result of a secondary alteration of the structure due to fossilization, as 

 originally it was neither calcareous nor horny, but consisted simjily of spicules 

 imbedded in a mass of parenchym. 



To determine the question of the original constitution of the fibrous skeleton of 

 the Pharetrones it is necessary to study the structure in those specimens which have 

 suffered least from the effects of fossilization. In the specimens from Warminster, 

 for example, which have been already referred to, the skeletal fibres have apparently 

 been altered to a very slight degree, and they are wholly composed of spicules in 

 close approximation to each other and as closely interwoven together as the strands 

 of a rope. There is no indication of their having been originally loosely imbedded in 

 a mass of parenchym in the same manner as in recent Leucones, in which, according 

 to Haeckel *, the chief portion of the skeleton, consisting of the entire thickness of 

 the body-wall, is composed of irregularly scattered spicules. A similar structure to 

 that of the Warminster examples is also present in transparent sections of Sestro- 

 stomeUa from Vaches Noires. The entire fibre is composed of large and small 

 spicules in intimate contact with each other, and bordering the margins of the fibre 

 there is a definite layer of spicular bodies running parallel to it, thus showing very 

 clearly that the spicul ir constitution of the fibre is original, and is not due to 

 secondary alteration. It is true that in many specimens the fibre is now only 

 partially composed of spicules imbedded in a transparent ground-mass ; but this 

 condition results from a partial dissolution of the spicular components of the fibre. 

 Various stages of this alteration may be traced in a series of specimens : in the best- 

 preserved the fibre is entirely composed of spicules, whilst in those which have been 

 most changed no spicules at all are present, and the fibre is destitute of organic 

 structure. I do not think therefore that Dunikowski's theory of the original simi- 

 larity of the skeleton of the Pharetrones to that of the recent Leucones can be 

 sustained ; and whilst admitting the great resemblance of the individual spicules in 

 these two groups, the fibrous arrangement of the skeleton in the Pharetrones appears 

 to me sufficient grounds for placing the group, as Zittel has done, in an independent 

 family. 



A short reference may here be made to the characters of the Dermal layer (Dermal- 

 schicht, Zitt. ; Deckschicht, Lunik.) in the Pharetrones, of which two distinct kinds 

 are present. One kind consists of a thin open layer of relatively large four-rayed 

 spicules, disposed on the outer surface of the sponge in such a manner that three of 

 the rays are parallel to the surface, whilst the fourth extends inwardly at right 

 angles to it. It is only under very favourable circumstances that this kind of dermal 

 layer has been preserved, and its absence in the specimens examined by Zittel led 

 him to suppose that it might be a distinguishing peculiarity of the group. It occurs 



* Die Kalkschwiimme, vol. i. p. 304. 



