SUB- GENUS IIETEROTRYPA. 117 



zontal. In the small corallltes the tabular are more numerous 

 and closely set. 



Obs. — Superficially, this species is readily distinguished by 

 its minute size and the numerous well-defined monticules which 

 cover the surface (PI. I. fig. 4) ; while its internal structure is 

 nearly as characteristic. Tangential sections (PI. I. fig. 4^) 

 show, as does the surface, that the corallum is composed of 

 two distinct sets of corallites, large and small, the latter more 

 or less completely encircling the former. The large corallites 

 differ in different parts of their course. In the central region 

 of transverse sections (PL I. fig. j\c) they are thin-walled and 

 entirely angular, still having a number of smaller tubes inter- 

 calated amongst them. In die outer portion of their course, 

 however, they become moderately thick-walled, and oval in 

 shape, their average diameter being, in this region, about 

 i-6oth inch. As, however, they widen out much on approach- 

 ing the surface, they have a much smaller diameter in the 

 interior of the branches. Each of the large corallites is sur- 

 rounded by a distinct wall, and they are hardly or not at all 

 in contact with one another, the interspaces between them 

 being occupied by a well-developed series of small, more or 

 less angular interstitial corallites, the size of which is variable. 

 Longitudinal sections (PL I. fig. \d), as transverse ones, show 

 that the corallites are remarkably straight during the whole of 

 their course, the extent to which they bend outwards as they 

 near the surface being exceedingly slight. The result of this, 

 as seen in transverse sections (PL I. fig. 4^:), is, that the central 

 axis of the corallum bears an unusually large proportion to the 

 circumferential zone. In the centre of the branches the tabulae 

 are comparatively few in number, but they become more 

 numerous as the surface of the corallum is approached, while 

 the walls of the tubes become at the same time thickened. 

 The tabulse are always complete, but the small interstitial 

 tubes differ structurally from the large ones in possessing much 

 more closely-set partitions than the latter. 



In its internal structure this species very nearly approaches 



