188 PROF. H. A. NICHOLSON AND DR. J. MURIE ON THE 



states of fossilization, from the Lower and Upper Silurian and the 

 Devonian rocks of Canada and the United States, specimens 

 from the Upper Silurian and Devonian of Britain, and a few 

 from the Upper Silurian deposits of Sweden. The great ma- 

 jority of these specimens have been personally collected by us, so 

 that we are able to speak with precision as to the exact gisement 

 of the specimens, and as to the condition of fossilization of the or- 

 ganic remains associated with them. A large number of the 

 microscopic sections have also been personally prepared by us, 

 and we are thus enabled to indicate with certainty their precise 

 relations to the specimens from which they are taken*. 



History and Literature. 



As regards the history of the genus, the following are the 

 principal works with which we are acquainted, or to which we 

 have been able to obtain access during the course of our in- 

 vestigations, excluding memoirs wholly concerned with descrip- 

 tions of species. 



The genus Stromatopora was originally founded by Groldfuss 

 (' Petrefacta G-ermanise,' 1826), and it was based upon the form 

 which has been generally known as Stromatopora concentrica. 

 At a subsequent page of the same work he describes another 

 form under the name of S. polymorpha, and defines the genus, re- 

 ferring it to the Sponges. It is, however, now certain, as Eoemer 

 has shown by an examination of the original specimens of Goldfuss 

 (' Lethaea Palseozoica,' 1877, explanation to pi. xxvi.), that these 

 two species are identical ; and though the latter form is described 



* As this is a joiut communication, it may be well to define to some extent 

 how far each of the authors is responsible for particular parts of the work. One 

 of the present writers (H. A. N.) had already devoted considerable attention to 

 the Stromatoporoids, having published several memoirs on the group, and 

 having arrived at tolerably definite views as to their structure and affinities, 

 though these views were not based upon the examination of thin sections with 

 the microscope. The great bulk of the material has also been collected and 

 worked out by the writer just alluded to. On the other hand, his colleague 

 brought to bear upon the subject a mind free from all preconceptions and pre- 

 possessions ; and the entire question of the structure and affinities of this most 

 difficult group of fossils has thus been most carefully debated and discussed be- 

 tween the two writers. This has involved extensit^e research on a number of 

 collateral points, wherein each has furnished his quota. Nor need we shrink 

 from stating that as fact after fact was accumulated, each has been obliged to 

 shift his ground more than once before arriving at a final decision, the difficul- 

 ties, as is so often the case, presenting themselves concomitantly with the increase 

 of our knowledge. 



