i8o JV. I. Robinson, 



phyllia. The generic characteristics of Hexaphyllia are, there- 

 fore, the prismatic to cyUndrical corallites, having thick walls 

 and regularly spaced septa that unite in the center. The chief 

 difference from Heterophyllia is the regular arrangement of thc; 

 septa. Completely formed tabulae occur in Hexaphyllia pris- 

 matica. Stuckenberg adds, ''Heterophyllia m'coyi and Hetero- 

 phyllia lyelli evidently belong in this genus." 



Forty specimens of Hexaphyllia cf. m'coyi from the Scottish 

 Lower Limestone Group, Gilnockie, near Canonbie, Scotland, 

 have been examined. They are all broken segments of corallites, \ 

 prismatic or cylindrical in shape. Many of them, especially the 

 larger ones, show the internal structure clearly when polished 

 with emery powder. They correspond closely to the definition of 

 the genus as given by Stuckenberg, but their reference to 

 Hexacoralla is not so clear. 



The adult corallites show six septa quite equally spaced but 

 younger stages indicate that their mode of introduction is that 

 of Tetracoralla. Such a stage is shown in PI. I, Fig. 9. The 

 bilateral symmetry is plainly indicated and the septal arrange- 

 ment clearly suggests the occurrence of an earlier stage of 

 growth with but four septa. However, even though the size 

 of the specimens is very inconstant, there is no section in the 

 many that were made that shows only four septa; on the other 

 hand, in several adult specimens the six septa meet at equal 

 angles at the center. Several of the smaller sections, however, 

 showed the junction of the third and second pairs of septa con- 

 siderably farther from the center than in the specimen shown in 

 PI. I, Fig. 9. In these the arrangement is more symmetrical and 

 it often is difficult to tell which arm of the Y formed by the 

 septa of the second and third bilateral pairs is the younger. The 

 number of septa is not large enough to show whether or not 

 the addition of septa obeys the quadriseptal rule of Faurot. The 

 arrangement of these six septa is that so characteristic of Tetra- 

 coralla. The arrangement of septa in these Scottish specimens 

 of Hexaphyllia therefore points to a relationship to Tetracoralla. 



Among the specimens of Hexaphyllia there were found sev- 

 eral extremely small cylindrical individuals with thick walls and 

 without septa which were at first thought to be young individuals 

 of this genus, but as there were no gradations in structure 

 between them and the six-septal forms, although the latter showed^ 



