GENERAL AND COMPARATIVE STRUCTURE. 35 



the great fattening of the branches, by their becoming palmate, 

 or by their mutual anastomosis ; and in one form (yJ/. briarca, 

 Nich.) there is no evidence that the base was fixed to a foreign 

 body. The dendroid form of corallum may, however, be re- 

 garded as the commonest of all the types of figure assumed 

 by the Monticuliporoids ; and it is exhibited by M. 7'amosa, 

 D'Orb., M.pidchclla, E. and H., M. hmiida, Phill., M. gracilis, 

 James, AI. Andrewsii, Nich., M. Jaincsi, Nich,, and very 

 many other species, the internal structure of these bearing no 

 relation at all to their externally similar shape. 



{d) The laminai^ or frondescent form of corallum, which 

 consists of a more or less widely expanded and laterally 

 compressed frond, which is rooted at its base, and has its 

 entire free surface covered with the calices. The corallum in 

 such cases consists of two strata of corallites, which are placed 

 back to back, so to speak, and diverge from a central plane to 

 opeii on the two opposed surfaces of the frond. The central 

 plane of the corallum may be marked merely by an irregular 

 cellular layer, or by an apparently definite, more or less com- 

 plete calcareous lamina, or more rarely {M. Dawsoni, Nich.) 

 the corallites are vertical in the median line of the expansion, 

 and simply diverge outwards as they grow upwards. The 

 forms of this type are usually constant in certain species, 

 though occasionally (as in M. molesta, Nich.) a normally lami- 

 nar species may assume a massive mode of growth. The spe- 

 cies which possess a frondescent corallum are M. luaiunnUata, 

 D'Orb., M. frondosa, D'Orb., AT. vwlesta, Nich., M. pavonia, 

 D'Orb., and M. Dazvsoni^ Nich. ; and it is noticeable that 

 while the form of the corallum is in these species remark- 

 ably similar, no two of them are at all closely related in their 

 internal structure. 



{e) The encrusting corallum, in which the colony consists of 

 very short corallites, forming a thin crust, which is parasitically 

 attached by the whole of the under surface to foreign objects. 

 The Monticuliporoids of this type are more like the ordinary 

 encrusting Polyzoa than are any others of the group ; and some 



