228 TABULATE CORALS. 



is enclosed by the epitheca only on its two free sides. In the 

 form known as H. escharoides, Lam., all the tubes are of ap- 

 proximately equal size ; but if, on the other hand, we make a 

 thin transverse section of an example of the typical H. catenu- 

 laria, Linn., we shall find a still more interesting and curious 

 condition of parts (PI. X., fig. 7, and PI. XI., fig. i). In these 

 cases the epitheca and proper walls of the corallites are directly 

 continuous (as in H. escharoides), but there is now the additional 

 feature that between each pair of the normal corallites there is 

 intercalated a much smaller sub-quadrate tube, which forms the 

 medium of union between the former. This interstitial tube, 

 moreover, does not seem to be bounded laterally by an inward 

 prolongation of the walls of the large tubes (as one would 

 expect it to be), but it appears to be enclosed by a proper 

 and peculiar wall of its own on the two sides where the large 

 tubes on either side come against it ; and this proper wall is 

 at once distinguished under the microscope from the wall of 

 the large tubes by its much darker colour and seemingly differ- 

 ent texture (see PI. XI., fig. i). There are thus shown to 

 exist two distinct sets of corallites in H. catenularia, Linn., 

 which occupy fixed and invariable relations to one another, 

 and can be proved by long sections to possess a marked differ- 

 ence in internal structure. Thus the large or normal corallites 

 of this species (PI. X., fig. 8, and PL XI., i a) have curved 

 or nearly straight complete tabulae, regularly and comparatively 

 remotely disposed. On the other hand, the small interstitial 

 tubes are intersected by much more numerous and more closely- 

 set tabulae, which are sometimes straight and sometimes sub- 

 vesicular, the condition of parts thus closely resembling what 

 is observed in the large and small corallites of Heliolites and its 

 allies. 1 In Haly sites escharoides ; as we have seen, the small 



1 So far as I know, Hall first noticed the occurrence of the small closely tabulate 

 tubes between the larger ones, as he says, in his description of H. agglomerata, that 

 the " spaces between the tubes" are " cellular" (Pal. N.Y., vol. ii. p. 129, 1852), and 

 he clearly figures the closely tabulate intermediate tubes. The first clear and at all 

 complete account of this subject appears to have been given by Fischer-Benzon in a 

 paper, " Ueber Halysites," to which I have unfortunately been unable to obtain 



