SUB-GENUS HETEROTRYPA. iii 



which are placed at intervals of from half a line to a line, 

 and are not occupied by coralHtes of specially large or small 

 dimensions. Calices sub-polygonal, with slightly thickened 

 margins, about i-QOth inch in long diameter, surrounded by 

 the openings of numerous small interstitial tubes. CoralHtes 

 conspicuously divided into two series, the small ones being 

 very numerous, and surrounding the larger tubes in a single 

 row, sometimes completely isolating the latter, and being 

 exceedingly variable in shape and size. In internal structure, 

 both sets of coralHtes are traversed by complete horizontal 

 tabulae, which are, however, much more numerous in the small 

 tubes than in the large ones. Walls thickened towards their 

 mouths, and apparently fused with one another. 



Obs. — The external characters of this species (PL II. figs. 

 2, 2a) are so well known that they require no further remark 

 here, its numerous conical monticules and the abundance of 

 the interstitial coralHtes being sufficiently distinctive features. 

 As regards internal structure, thin tangential sections (fig. i8, 

 A, B, and c) exhibit in a striking manner the conspicuous 

 division of the coralHtes into two sets of tubes, large and 

 small, and the great development of the latter. The large 

 tubes are very uniform in size, generally oval or circular in 

 shape, and moderately thick-walled — the thickening of the 

 wall, however, never proceeding to the extent that obtains in 

 forms such as AI. {Heterotrypa) gracilis, James, AI. [Hclero- 

 trypa) tninida, Phill., and allied types. The small coralHtes 

 are very variable in size and form, and are principally de- 

 veloped at the angles of junction of the large tubes ; but they 

 are commonly so numerous as to form a complete zone round 

 the large coralHtes, though such a zone never consists of more 

 than a single row. In tangential sections taken just below 

 the surface (fig. i8, c), each of the large coralHtes is seen to 

 be surrounded by a well-marked ring of sclerenchyma, all the 

 tubes, however, being firmly united together. Between the 

 boundino-rinofs of the laro-er coralHtes are situated the small 

 tubes, which, in sections of this nature, are mostly oval or 



