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(h) Axis. — The axis is horn)^ and in most cases slender and flexible, e.ff., 

 M. gracilis, Wright and Studer ; M. crassa, Wright and Studer, and 31. flexilis, 

 Hiles ; but in M. ramosa, Thomson and Henderson, especially in the older 

 parts, it is very hard and rigid. It is sometimes chambered in the younger 

 portions and is faintly striated in M. hengaleims, Thomson and Henderson. 

 Typically it is cylindrical, but in the older parts of M. rubra, Thomson, M. 

 complanata , Wright and Studer, and M. flerilis, Hiles, it is flattened in the 

 plane of branching, while in M. ramosa, Thomson and Henderson, the flatten- 

 ing takes place in the plane perpendicular to that of ramification. 



(r) Cdenenchyma. — In most cases the coenenchyma is thin and transparent ; 

 it contains a single layer of spicules, or an inner continuous layer of small 

 spicules and an outer discontinuous layer of larger spicules arranged longitudin- 

 ally. In M. crassa, Wright and Studer, it is thick and dense and contains 

 several layers of spicules, being in some places twice as thick as the axis. In 

 M. umbraticoides, Studer, it is thin ; in M. tenera, Eidley, thin and paper-like ; 

 in M. rubra, Thomson, it is thin and almost smooth. In M. [iracilis, Wright 

 and Studer, it is comparatively thick but fairly even, while in M. beiigalensis, 

 Thomson and Henderson, it is thick and rough. In M. ramosa, Thomson and 

 Henderson, it is thin, almost transparent and very rugose, while in M. arborea, 

 n. sp., it is thick and very irregular in contour. The nature of the surface is 

 often characteristic, as may be seen by contrasting that in M. rubra, Thomson, 

 and M. purpurea, Whitelegge, with that in M. ramosa, Thomson and Henderson, 

 and M. arborea, n. sp. Another noteworthy feature is that while the spicules 

 in the younger branches and twigs are often very large and longitudinally 

 arranged, those in the main stem and larger branches are much smaller and 

 show no definite arrangement. This is most noticeable in M. raino.'^a, Thomson 

 and Henderson, M. rubra, Thomson, and M. arborea, n. sp. The fact must 

 be borne in mind in identifying a species from a fragmentary portion of a colony. 



(d) Polyps. — The disposition of the polyps shows considerable diversity. 

 In the majority of the species, especially on the twigs, they are arranged on the 

 lateral surfaces, but in M. complanata, Wright and Studer, they occur on three 

 surfaces. In M. ramosa, Thomson and Henderson, they arise on the surface of 

 the main stem and larger branches in some specimens, while in others this 

 surface is quite bare ; on the twigs they are lateral and sub-alternate. In M. 

 nitida, Verrill, they occur mainly on one surface, while in M. tenera, Kidley, and 

 M. perramosa, Wright and Studer, they are lateral and alternate. In M. 

 gracilis, Wright and Studer, and M. crassa, Wright and Studer, they are dis- 

 posed spirally on the thickened ends of the branches. The ends of the twigs 

 are generally occupied by two almost opposite polyps with between them a 

 small projection consisting of the tip of the axis covered by canienchyma, e.g., 

 M. tenera, Eidley, M. flejcilis, Hiles, M. purpurea, Whitelegge, and M. arborea, 



