32 BRITISH STROMATOPOROIDS. 



ohioensis 1 . In this specimen all the interspaces of the fossil have been filled in 

 with a fine-grained greenish calcareous mud, the skeleton having been subsequently 

 dissolved out and then replaced, more or less completely, with transparent calcite 

 (PI. II, figs. 1 and 2). Another case of the same mode of preservation is presented 

 by the curious fossil described by Dr. Murie and myself from the Trenton Lime- 

 stone of Canada, under the name of Stromatocerium canadense (' Journ. Linn. Soc.,' 

 p. 223, PL iii, figs. 9 and 10). At the time we described this fossil, we had not 

 observed any cases of the mode of preservation now in question, and we therefore 

 naturally regarded the portions of the fossil which were composed of calcite as 

 being the canals and chambers of the organism, and the dense and opaque portions 

 as being the skeleton. In reality, however, the chambers have been filled with 

 dense calcareous mud, and the skeleton has been replaced by calcite (PI. II, figs. 

 3 — 5). Stromatocerium canadense, Nich. and Murie, when viewed in this light, is 

 therefore no longer the anomalous form that it appeared to be, but is readily 

 recognised as a species of Labechia, which, being apparently distinct from 

 previously described forms, must stand as L. canadensis, Nich. and Murie. 



3. Minute Structure of the Stromatoporoids. 



All palaeontologists probably will readily admit that the study of the Stromato- 

 poroids can only be prosecuted, with any certainty, by means of properly prepared 

 thin sections, which can be examined under the microscope by means of transmitted 

 light. In mode of growth, in their general form, and in their merely superficial 

 characteristics, many Stromatoporoids present a remarkable similarity ; and hence 

 many observers have been led to regard the majority of these organisms as being 

 nothing more than variations of a common type, their differences being supposed 

 to be due to local conditions, or to the adaptations rendered necessary in different 

 individuals by differences in their environment. Thus, even at the present day, so 

 distinguished and acute an investigator as Professor Ferd. Roemer is inclined to 

 regard the greater number of the Devonian Stromatoporoids of Germany as mere 

 variations of the Stromatopora concentrica of Goldfuss, and a considerable number 

 of the Upper- Silurian forms as variations of S. striatella, D'Orb. 



My own experience, based on a study of many hundreds of microscopic slides, 

 has led me to the conclusion that the minute internal structure of the skeleton of 

 the Stromatoporoids shows very remarkable and constant differences, even in types 

 which in external aspect are not very dissimilar ; that in properly prepared sections 



1 So far as I am aware, no species of Labechia has been hitherto recorded from the Silurian Bocks 

 of North America. Labechia ohioensis differs from L. conferta, Lonsd., in the greater delicacy of the 

 radial pillars, these structures often appearing to be angulated rather than round, while the vesicles of 

 the interstitial tissue are much more minute than in the latter species. 



