STROMATOPORA CONCENTRICA. 167 



structure, oval or circular clear spaces separated by dark intervals marking out the 

 lines of the principal tubes (Plate XXIV, fig. 9). This curious phenomenon is 

 easily recognised in polished sections of this form, even by the naked eye, or with 

 a lens, but I cannot give any satisfactory explanation of it. The skeleton in this 

 form grows in " latilaminse," as in the ordinary examples of the present species. 

 The large size and characteristic form of the astrorhizas in this type might, how- 

 ever, perhaps justify us in considering it as a distinct species of Strmiiatopora 

 rather than as a mere variety of 8. conceutrica, Goldf. 



Specimens of all the three forms of 8. conceutrica above distinguished commonly 

 occur in the " Caunopora-state." In such specimens the " Caunopora-tubes " are 

 generally of very small size, usually about \ mm. in diameter, but sometimes 

 reaching a diameter of § mm. The tubes are connected with one another by 

 horizontal stolons, as is the case with " Caunopora-tubes " generally, but I have 

 not recognised in them any structures of the nature of " tabula?," nor do they 

 appear to be provided with septal spines. 



S. conceutrica, Goldf., is more or less nearly related to 8. Garteri, Nich., and 

 8. discoidea, Lonsd., and, in a less degree, to 8. Hupschii, Barg., in all of which 

 the skeleton-fibre is thick and coarse. From 8. Hupschii the present species. is 

 readily distinguished by the much less open reticulation of the skeletal frame- 

 work, while the radial pillars and zooidal tubes are not so regular nor so well 

 developed. The skeleton-fibre of S. Hupschii is, moreover, even more coarse than 

 that of 8. conceutrica, while its cosnosteum does not grow iu latilaminse. 8. dis- 

 coidea, Lond., also has a thicker skeleton-fibre than that of S. conceutrica (j to 

 j mm. in diameter as compared with -g- to £ mm. in the latter) ; and is at 

 once distinguished from the present species by its extraordinarily developed 

 astrorhizal system. 8. Carteri, Nich., again, grows in latilaminaa, but the skeleton- 

 fibre is of a finer character than that of 8. .conceutrica (about ^ or y mm. in 

 diameter) and the general reticulation of the skeleton is much more lax and open. 

 Moreover, the skeleton-fibre of 8. Garteri is more coarsely porous than that of 

 8. conceutrica. Lastly, S. tijpica, Rosen, has a skeleton-fibre of about \ mm. in 

 diameter, and the general ccenosteal tissue is much less dense than that of 8. con- 

 ceutrica, while the radial pillars and tabulate zooidal tubes are much better 

 developed than in the latter species. 



I have found it impossible to draw up a satisfactory synonymy of this species, 

 owing to the great difficulty of determining the real nature of many of the forms 

 described under this name by older writers. The 8. conceutrica of Michelin 

 (' Iconographie Zoophytologique,' p. 190, pi. xlix, fig. 4, 1840 — 47) is quoted 

 from both Devonian and Silurian localities, and thus clearly cannot be relied 

 upon ; and though his figure might answer very well for that of a fragment of 8. 

 conceutrica, Goldf., it would stand even better for one of Actinostroma stellulatum, 



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