G4 Mr, J. Thomson and Dr. H. A. Nicholson on the 



be, however, it is clearly separated from Cyathophyllum by 

 the absence of any external area of vesicular tissue. 



The genus HdioiDliyllum^ Hall, though in certain respects 

 nearly allied to Gyathophyllum, and appearing in great part to 

 take its place in certain formations, is nevertheless distin- 

 guished by characters of primary importance. It agrees with 

 Cyathophyllum in possessing a circumscribed central tabulate 

 area, in the extension of the septa to the centre of the visceral 

 chamber (where they are more or less twisted and coalescent) , 

 and in the fact that the circumferential zone of the corallum is 

 more or less minutely subdivided into cells by the development 

 of dissepiments in the interseptal loculi. With these sub- 

 stantial points of agreement, there is the following striking- 

 dissimilarity of structure : — In Cyathophyllum the lines of dis- 

 sepiments run from the theca inioards and doivnwards^ so as 

 to form a series of layers of minute vesicles having a corre- 

 sponding inclination. In Heliophyllum^ on the other hand, the 

 interseptal loculi are divided into compartments by the inter- 

 section of two sets of dissepiments, of which the primary and 

 far most conspicuous series is directed from the internal 

 surface of the wall obliquely inioards and upwards, towards 

 the centre, in a succession of ascending arches, the convexities 

 of which are directed upwards. The dissepiments of this series 

 appear on the free edges of the septa within the calice as so 

 many short spines ; and they communicate to the sides of the 

 septa, as seen in transverse sections, a characteristic and un- 

 mistakable denticulation. They are intersected, generally 

 nearly at right angles, by a second series of dissepiments, 

 which are much more delicate, more disconnected, and more 

 variable than the preceding, but which generally run inwards 

 and downwards from the wall. 



The genus Omphyma, Rafinesque and Clifford, is, again, 

 related to Cyathophyllum, the central tabulate area being sur- 

 rounded by an outer zone of large vesicles having an upward 

 and outward direction. It is, however, distinguished by the 

 fact that the septa do not coalesce centrally, bitt leave a small 

 portion of the tabulae free to view, by the presence of four 

 shallow septal fossulee, by the possession of root-like out- 

 growths of the epitheca, and by the comparatively gigantic 

 size of the vesicles filling the outer portion of the interseptal 

 loculi. 



Finally, we may briefly consider the forms which have been 

 at various times placed under the names Caninia and Cya- 

 thopsis. Most of the forms included under the genus Caninia, 

 Mich., have been shown by Milne-Edwards and Haime to 

 appertain in reality to Zaphrentis. This is the case, more 



