MINUTE STBTJOTUEE OF STKOMATOPORA AND ITS ALLIES. 217 



are common to forms wliicli in intimate structure are very 

 different. Thus the radiating surface-canals which induced 

 Wiuchell to found the genus Coenostroma are found in forms 

 having the intimate structure of Stromatopora, in the utterly 

 abnormal Pachystroma, and (in an especially well-marked form) 

 in the group of Stromatoporoids designated by one of us Syringo- 

 stroma (Nicholson, Pal. Ohio, vol. ii.). It appears, therefore, 

 that we must fall back in classifying the Stromatoporoids upon 

 their ultimate constitution as shown by the microscope, dis- 

 carding such general features as the presence of radiating surface- 

 grooves and w^ater-canals, and relying solely upon the different 

 modes of arrangement of the minute elements of the skeleton. 

 In this case, such genera as Coenostroma, Winch ell, and Syringo- 

 stroma, Nich,, will have to be abandoned and redistributed, or 

 possibly retained as subgenera. In the followiug pages we shall 

 bring forward a provisional arrangement of the Stromatoporoids, 

 based upon their minute structure, and excluding in the meanwhile 

 all types which do not appear to us to be easily recognizable. 



{a) Steomatopora, Goldfuss. 

 (Petrefacta G-ermaniae, 1826.) 



Skeleton (" sarcodeme ") consisting of concentric calcareous 

 laminae, separated by distinct " interlaminar spaces," which are 

 crossed by numerous vertical "radial pillars." In some cases 

 there are radiating water-canals and surface-grooves placed round 

 minor centres. Sometimes there are seen on the surface the 

 openings of large water-canals (" oscula "). 



Habit. — Forming irregular masses, sometimes with a foreign 

 body as a nucleus ; spreading out into extended expansions, 

 covered inferiorly by a thin striated calcareous membrane (" epi- 

 theca"), or growing in thin layers parasitically upon foreign 

 objects. 



Type. — Stromatopora polymorplia, Goldfuss. 



"We have not personally had the opportunity of examining 

 specimens of 8. polymorpha, but judging from the figures and 

 descriptions of authors, and especially from those of Von Rosen, 

 there can be no doubt that its minute structure is as indicated in 

 the above definition. If this be the case, then Stromatopora 

 proper will include all those Stromatoporoids in which the skele- 

 ton consists of concentrically disposed horizontal laminae, the 



