Dr. H. B. Boll— On Fossil Sponges. 351 



horizontal rods or fibres, whicli become more abundant in the Oolitic 

 and Cretaceous rocks, and have their representatives even in the 

 present day. The AmorphospongidcB first make their appearance in 

 the Silurian rocks, and occur more or less abundantly in the cal- 

 careous marine deposits of all the succeeding epochs ; and species 

 are still living in our present seas, for which, as far as external 

 appearances are concerned, at any rate, it is difficult to find good dis- 

 tinctive characters. The cup-shaped and cylindrical forms of this group 

 commence in the Devonian and Carboniferous Limestones, and in the 

 Mortiera vertehralis (De Koninck)/ we have a depressed form of the 

 latter, which, in the Mountain Limestone (?) of India attained 

 greater vertical development. There are recent forms which, to all 

 appearance, are undistinguishable either in figure or in the texture of 

 the rete, and the only appreciable difference that can exist must be 

 in the structure of the fibre. Siphonia pyriformis is apparently a 

 still living species, well-preserved specimens from Blackdown pre- 

 senting no external character to distinguish them from the recent 

 form, nor with certainty do its structural details. The Warminster 

 specimens are seldom well preserved ; but in the flints of the Chalk 

 thin sections sometimes show the spicular structure of the cords, 

 of which the skeleton of this sponge is chiefly composed. 



All the fossil sponges, exclusive of those masses of scattered 

 spicula found in the Mountain Limestone Chert of the Great Orme's 

 Head, the Lias of Glamorganshire, or the flints of the Chalk, etc., 

 appear to be capable of being arranged in four groups having a 

 common character — viz. 1st, those in which the skeleton is built up 

 mainly of fibres or elongated spicula, which cross each other more 

 or less at right angles, but which, in the cylindrical forms of this 

 group, assume in part a radiating arrangement ; 2nd, those in which 

 it is constituted of variously formed spicula, heterogeneously 

 arranged ; 3rd, those in which the skeleton consists of a rete, the 

 cords of which are formed of spicula ; and, 4th, those formed of a 

 rete of fibres in which spicula, if present, were only accessory, and 

 which, judging from the general structure of the fabric, were 

 probably keratose or horny sponges. No doubt the first two groups 

 trench upon each other, in so far that the rectangular structure is 

 frequently accompanied by accessory stellate and other spicula ; and 

 the last two may be often difficult to difi'erentiate, in consequence of 

 the structure not being sufficiently well preserved. These, however, 

 are difficulties which the palaeontologist has to contend with constantly, 

 and which it is his object, with time and opportunities, to remove. 

 Many a fossil conchifer has been moved from genus to genus, until 

 the structure of its hinge was ascertained ; many a mollusk is still 

 uncertain as regards its affinities to existing genera. But on their 

 relation to existing genera and species, which can be arrived at only 

 by patient inquiry into structural details microscopically, by means 



^ Placed doubtfully among the Zoant/iaria, by M. Edwards and Haime but 

 specimens of this fossil from the Great Orme's Head, better preserved than De 

 Koninck's types, now ia the British Museum, enable the author to assign them a place 

 among the sponges. 



