38 MR. J. THOMSON ON THE OCCURRENCE OF DIPHYPHYLLTJM 



points rise from the surface of a tabula, and when there are rods 

 they transfix, as it were, several tabulae. The rods are rarely seen, 

 but, by searching, some will be found in a somewhat definite per- 

 centage of corallites. The triareal nature of the corallites is to be 

 recognized, but it is a term which has become disused, especially as 

 the endotheca only gives characters of second-rate importance as a 

 rule. 



The fissiparity is much better shown in the specimens herein de- 

 scribed than it was in those seen by Lonsdale, and there are three kinds 

 of the process. In some corallites bending in and figure of 8 occurs, as 

 in the Mesozoic and Recent Corals, and division took place at the nar- 

 rowing. But usually a ridge grows across the corallite, and septa are 

 formed on either side, and then the ridge, which, for a time, has been 

 partly the wall, separates into two portions. The third method is 

 singular, for two ridges grow towards the centre of a corallite, and 

 one reaches the other at right angles near the axis, and thus the 

 appearance of a trilobed budding is presented ; but it is evident that 

 septa only grow from the ridges, and that would not be the case in 

 buds. After separating, the new corallites grew upwards away 

 from one another. The ridges, which have so much to do with the 

 two commonest kinds of fissiparity, are the extension inwards of 

 oppositely placed large septa ; the inner ends unite and shut off the 

 two parts of the corallite, and septa grow from the faces looking 

 towards the new central areas. In another form it appears as if a 

 tabula turned up or grew up at its outer edge and stretched across 

 the corallite at the calice ; it came up to the bottom of the visceral 

 cavity, and then septa grew from both sides of it and fission occurred. 



Fissiparous growth is a very rare phenomenon amongst the Rugose 

 Corals, and, so far as is known, Diphyphyllum is the only genus 

 in which it occurs. The alliances of the genus need hardly be 

 noticed here, as they have been discussed by Milne-Edwards and 

 Jules Haime, and especially by De Koninck in the work already 

 quoted. 



The presence in Scotland of a species of a genus which would come 

 within that which should receive De Koninck's D. concinnum (non 

 Lonsdale) has been discovered of late, and it necessitates the re- 

 definition and renaming of the Belgian type, so as to separate it from 

 the fissiparous form. This new genus will form the subject of a 

 future communication. 



EXPLANATION OF PLATES IV. & V. 

 Plate IY. 



Fig. 1. Diphyphyllum concinnum, Lonsd., transverse section showing fissiparity. 



2. ■ , var. furcatum. 



3. Blackwoodi, Thorns., transverse section. 



4. , var. appro ximatum, transverse section. 



o. cylindricum, Thorns., transverse section ; 5 a, longitudinal section 



of corallites. 



