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



of the calyx, the ventral surface forming a narrow space, which is either naked 

 or is covered with small scales. The coenenchyma usually conta.ns small elongated 

 scales. 



3. Pt-imnoa, Lamouroux, Hist, des pol}'p. Hexibles, p. 440, 1816; Expos, method. 



de rOrdre des polypiers, p. 37 ; Milne-Edwards, Hist. Nat. des Coralliaires, 



t. i. p. 139 {ixirs); Gray, Cat. Lithophytes Brit. Mas., 1870, p. 44. 



Prijmnoa, Elirbg., Corallen. des rothen Meeres, 1834, p. 133. 



Lithoprimnoa, Grube, Abhandl. d. schles. Ges. Xaturw. u. Med., 1861, p. 165. 



The colony is branched. The polyps occur in close spirals over the entire periphery 

 of the stem and branches. Each polyp calyx is enclosed dorsally and laterally by two 

 large scales, of which there are two longitudinal overlapping rows, the upper margin 

 of one scale always overlapping the lower border of the next. There is a ventral 

 space destitute of scales, except at the calyx margin, where four are present, forming 

 with the two pairs of dorso-lateral scales an operculum. The calyces can be turned 

 downwards from the stem. 



4. Stachyodes, Wright and Studer, Archiv f. Naturgesch., Jahrg. liii. Bd. i. p. 49. 



The colony is but feebly branched. The calyces arise in regular whorls of five 

 from the stem and branches. The scales of the polyp calyces are disposed in a 

 manner analogous to that observed in the preceding genus. The polyp calyces when 

 at rest are turned downwards. 



.5. Ccdypterinus, "Wright and Studer, Archiv f. Naturgesch., Jahrg. liii. Bd. i. p. 49. 



The colony is simple and rod-like. The polyp calyces occur in whorls of five to 

 seven around the stem. The polyps do not, however, surround the whole stem, but 

 leave on one side a space free from polyps. Since this occurs in the same position 

 in all the whorls, a region comparable to that in CaUozostron occurs along the entire 

 stem.. In contrast to the portion containing the pol}-ps, the space appears as a deep 

 groove, which is formed into a canal by the curvature round it of the expanded 

 margins of a scale from the adjacent polyp calyces on either side. The polyp 

 calyces are covered by dorsal and lateral scales arranged in two longitudinal rows. 

 The two basal scales of these rows are very large and wing-like, above these are two 

 median scales, and then the superior pair which support the opercular scales. 



6. SteneUa, Gray, Cat. Lithophytes Brit. Mus., p. 48, 1870. 



The colony is sometimes simple, but usually branched ; the pol}"p calyces are large, 

 and arise from the axis at right angles either in opposite pairs or in whorls of three. 



