Dr. H. A. Nicholson on the Genus Stromatopora. 5 



sheet, made up, like the preceding, of successively superim- 

 posed laminEe. 



The main element, therefore, in the structure of Stromato- 

 l)ora, and the only one about which all observers are tolerably 

 agreed, is a system of parallel calcareous laminas, generally of 

 great tenuity, not in actual contact, but separated from one 

 another by narrow interspaces. The successive lamina3 are 

 kept apart by a vertical system of calcareous pillars, which 

 divide the interspaces between the lamina3 into minute, usually 

 quadrangular compartments, and thus render the whole mass 

 more or less minutely vesicular. 



So far, the stmcture of Stromatopora would be compatible 

 with a reference of the genus either to the Foraminifera or the 

 Spongida; but there are unfortunately many differences of 

 opinion as to the fui-ther structure of these fossils, and these 

 have led to equally wide differences of opinion as to the affini- 

 ties and systematic position of the genus. 



According to M'Coy (Pal. Foss. p. 12) the vesicular tissue 

 of Stromatopora is composed of " minute, curved, calcareous 

 plates," which he compares to the coenenchyma QiPalceopora 

 and Flstidipora. He also states that the upper surface is oc- 

 casionally marked " with extremely obscure, distant, quincun- 

 cially arranged, small pits," which he appears to think may 

 represent the corallites in the above-mentioned or other allied 

 genera. 



Prof. Hall agrees with M'Coy in referring Stromatopora to 

 the Coslenterata, and in placing it in the neighbourhood of 

 Tuhipora (Pal. N. Y. vol. ii. p. 135). He considers that the 

 fossils of this genus are composed of " minute cylindrical tubes 

 with considerable space between, and that the laminated struc- 

 ture arises from thin layers of calcareous matter, deposited and 

 filling the spaces between, and enclosing the tubes." 



My own investigations of a very extensive series of speci- 

 mens from the Lower and Ujjper Silurian rocks and from tlic 

 Devonian Formation have led me to the conclusion that the 

 genus Stromatopora is clearly referable to tlie Spongida, and 

 that it should be placed amongst the Caleispongia?, a group 

 represented by many and varied forms both in past time and 

 at the present day. The reasons for this belief may be shortly 

 summed up as follows : — 



a. The fundamental structure of Stromatopora is by no 

 means inconsistent with the belief that it belongs to the calca- 

 reous sponges. It does not consist of reticulated calcareous 

 spicula, as in the more typical members of the group ; but 

 neither does it consist of a vesicular tissue coinjioscd of " minute 

 curved calcareous plates " (M'Coy), which could be in any way 



