60 THE VOYAGE OF H.M.S. CHALLENGER. 



given above fairly agree with this species, with the exception of the distinction of a 

 simple corallum. Since the only example hitherto known is a fragment, no conclusion as 

 to the entire colony could be drawn from it. Thouarella antarctica, Val., is represented 

 in the Challenger collection, but there is also a series of nearly related forms, which 

 constitute, with the first species, a genus for which the name Thouarella may be retained. 

 The diagnosis of the genus has had, however, to be essentially modified. 



The colony consists of a main stem, which is generally simple, seldom divided into 

 several branches, which then lie in one plane. From the main stem are given oif twigs 

 in difierent directions, generally in threes, which come off at successive heights at nearly 

 right angles. They are always much thinner than the main stem, and either remain 

 simple or give ofi" twigs, as does. the main stem. The polyps are large relatively to their 

 support, club- or cup-shaped, and are arranged on the twigs in short, ascending spirals of 

 three polyps each, seldom opposite. The ccenenchyma of the stem and twigs is thin, and 

 contains an outer layer of flat, scale-hke, calcareous spicules, which overlap one another 

 at their edges, and below this there are small triangular or polj-gonal scales. The 

 calyx scales are difierent on the dorso-lateral and on the ventral sides of the bilateral 

 calyx. The dorso-lateral scales have strong, convex upper edges, towards which the 

 Kttle prominences, radiating from the nucleus, diverge. Frequently the prominences 

 fuse, and then form continuous ribs which terminate in projecting teeth at the edge of 

 the scale, often the middle tooth of the uppermost row of scales is especially prominent, 

 at times forming a long spine. The ventral scales are only in one case similar to the 

 dorsal ones, otherwise they are small, thin plates, which generally form two longitudinal 

 rows, and which, on the ventral curvature of the calyx, may overlap one another with 

 their edges. The upper scale is then overlapped by the upper edge of the one next 

 below it. 



The operculum is formed of eight pointed curved scales, convex towards the outer side, 

 which laid together form a low cone. In the middle line on each opercular scale is found 

 a deep longitudinal furrow. When closed the lateral edges do not touch one another but 

 leave eight radial slits between. 



The growth of the stem and the development of the twigs takes place at the apex of 

 the colony, while at the same time the twigs at the base of the colony die ofi". A single 

 polyp forms the origin of the new tmg. The base of the polyp elongates like a 

 peduncle, in which a thin axis is developed. First of all two new polyps bud forth on 

 the peduncle, which appear to be arranged in a short spiral. This mode of growth 

 also takes place in those forms which bear polyps arranged oppositely on the end of the 

 twig. 



The genus TJiouarella comes nearest to Stenella; the fijst species to be described, 

 Thouarella moscleyi, exhibits in the form of the calyx and in the structure of the colony 

 a close relationship to this genus, and may be regarded as an intermediate form. 



