CORALS AND CORAL-ANIMALS. 179 



The genus Madrepora represents the most extensive existing natural group of closely-allied 

 specific forms included in the coral class, and may in this respect be appropriately compared with 

 the specifically rich botanical genera, Rosa or Salix. As in the case of the botanical groups, con- 

 siderable difficulty attends the definition of the slightly-varying structural characters that serve 

 to distinguish the more nearly allied types, of which over two hundred varieties have been 

 described. The determination and identification of the number of species — no fewer than 

 seventy — that were collected by the author in the Great Barrier district proved by no means 

 a sinecure to the authority, Mr. George Brook, F.L.S., to whom they were consigned for 

 incorporation with the British Museum collection. Out of this considerable number, fourteen 

 have proved to be absolutely new species, while some ten or twelve others represent co-types 

 now described for the first time, in association with duplicate specimens recently received from 

 other localities of the Indo-Pacific area The enumeration of the characters of the seventy 

 specific forms obtained from the Great Barrier district lies outside the purpose of this 

 volume. A brief notice of some of the more conspicuous species can alone be attempted. 



As an aid towards the accomplishment of their systematic nomenclature, it is found con- 

 venient to arrange the numerous specific forms of the genus Madrepora into sections, whose 

 respective members correspond with one another in the characteristic growth-pattern of their 

 coralla. Thus, in one numerically abundant series, the coralla are distinctly arborescent, 

 consisting of lax irregularly-branching growths which, with reference to their more or less 

 distinct, antler-like contour, are popularly known as "Stags'-horn Corals." In a second series, the 

 branching coralla are so closely aggregated as to form symmetrical, shrub-like clumps. In 

 a third group, the base of the corallum expands in the form of a vase, the upper surface of 

 which gives origin to the short, polyp-bearing secondary branchlets. In a fourth series, the 

 branches either exhibit a bluntly lobate contour, or may be flattened out in the form of broad 

 folia. These suggested sectional divisions are necessarily artificial, and are introduced only 

 for the purpose of facilitating specific identification ; as they actually exist in nature, they are 

 intimately interwoven with one another through the interposition of connecting types. 



A feature that is common to all the numerous specific forms of the genus Madrepora is 

 represented by the structure of the associated polyps. These are of a very simple type. They 

 possess only twelve tentacles, and twelve mesenteric and septal cycles, or " sarcosepta " and 

 " sclerosepta," as they are technically designated. It is remarkable, however, that in not a few 

 of the specific varieties, one of the tentacles is invariably of abnormal size, being twice the length 

 of any other. It also commonly happens that the single terminal or growing corallite, and the 

 associated polyp, at the extremity of each branch or branchlet, is conspicuously larger than 

 those of the lateral series. This abnormally large terminal corallite and polyp, together with 

 all the adjacent ones that enter into the composition of the growing apices, differ, very fre- 

 quently, to a marked degree in colour from those of the preceding or more proximal area. 



A A 2 



