so Harveij B. Hall — On Fossil Spojiges. 



nui pyrlfonnis is apparer.tly a still living i^pecie^!, Avell preserved speci- 

 mens from Blackdown, presenting^ no external character to distinguish 

 them from the recent form, nor with certainty do its structural details. 

 The Warminster specimens are seldom Avell preserved ; but in the flints 

 of the chalk thin sections sometimes shi)\v the sjiicular structure of the 

 cords, of which the skeleton of the sponge is chiefly composed. 



All the fossil sponges, exclusive of those masses of scattered spicula 

 found in the mountuiu limestone chert of the Great Orme's Head, the 

 Lias of Glamorganshire, or the flints of the chalk, etc., appear to he 

 capable of being arranged in four groups, having a common character, 

 viz.: 1st, those in -which the skeleton is built up, mainly of fibers 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 radia- 

 tuig arrangement; 2d, those in which it is constituted of variously 

 formed spicula, heterogeneously arranged ; 3d, those in Avhich this 

 skeleton consists of a rete, the cords of which are formed of spicula ; 

 and 4th, those formed of a rete of fibers in which spicula, if present, 

 were only accessory, and which, judging from the general structure of 

 the fabric, were probably kcratose or horny s})onges. Xo doidjt the 

 first tv/o groups trench upoji each other, in so far that the rectangular 

 structure is frequently accompanied by accessory stellate, and other 

 spicula ; and the last iwo may be often difficult to differentiate, in con- 

 sequence of the structure not being sufficiently well preserved. These, 

 however, are difficulties which the paleontologist has to contend with 

 constantly, and which it is his object, with time and opportunities, to 

 I'emove. Many a fossil conchifer has been moved fj-om genus to genus, 

 until the structure of its hinge was a-scertained ; many a mollusk is still 

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

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

 patient inquiry into structural details microscopically, by means of 

 thin sections or otherwise, can the differentiation of the fossil spongia- 

 dae be satisfactorily made. Occa.<ionally the structure, especiallv in 

 the silicified sponges, is so admirably prepared as to render this not dif- 

 ficult ; but until their true affinities to recent species have been studied 

 from a str'icthj zoological point of view, our knowledge concerning them 

 nuist be wanting in scientific precision. The result of such inquiries 

 will probably be to reduce many genera to the lower grade of species, 

 and many species to mere varieties or conditions of growth. In com- 

 mon with other forms, equally low in the scale of organization, the 

 sponges appear to have endured through a long range of time, subject 

 only to modifications, which scarcely amount to specific distinctions. 



