Corals dredged by H.M.S. ' Challenger. 3 555 



develop pali. The corals have been placed provisionally in the genus 

 Ceratotrochus. New genera will possibly have to be formed for C. 

 'platypus and C diadema. 



Flahellum alabastrum, sp. n. 



The coralhmi is of a beautiful light pink colour, and is very thin and 

 fragile. It is wedge-shaped, with an attenuated pedicle. The lateral 

 costae are sharp, with slightly prominent irregular edges ; they make 

 an angle with one another of from 120° to 140°. The external surface 

 of the corallum is covered with a glistening epitheca. The primary and 

 secondary costae of the faces are almost as well marked as the lateral 

 costae, and appear as irregularly dentate ridges, separated by slight de- 

 pressions. The calicle is compressed from side to side in its centre. 

 Its upper margin is curved, describing about one third of a circle, and is 

 deeply indented, the primary and secondary costae being prolonged into 

 prominent pointed processes. There are six systems of septa disposed 

 in five cycles. The primary and secondary septa are, approximately, 

 equal ; the tertiary, quaternary, and quinary successively narrower. 

 The quaternary septa join the tertiary at a short distance before reaching 

 the columella. The septa of the fifth cycle are incomplete. Three per- 

 fect specimens obtained had about the same dimensions. Extreme 

 height of the calicle 5 centims., longest diameter 6-5 centims., shortest 

 diameter 3 centims. The complicated, sharply-toothed margin of the 

 calicle forms the most striking feature about this Flahellum. Its 

 extreme fragility is remarkable in so large a species. It appears to 

 belong to the division of the genus Flabella subpedicellata of MM. 

 Edwards and Haime, and to the subdivision of these characterized by 

 having the costae prominent and ridge-like on the faces, as well as on 

 the lateral margins. These corals, however, have the fifth cycle of septa 

 complete. Two of the specimens obtained were alive, and expanded 

 themselves when placed in sea-water, notwithstanding the depth from 

 which they came. The inner margin of the disk around the elongate 

 mouth presents a regular series of dentations corresponding with the 

 septa, and is of a dark madder colour ; the remainder of the disk is of a 

 pale pink. The tentacles take origin directly from the septa ; they are 

 of an elongate conical form ; those of the primary and secondary septa 

 are equal in size and placed nearest the mouth and at an equal distance 

 from it, together with the tertiary tentacles, which are somewhat shorter, 

 but are placed in the same row with them. The tentacles of the fourth 

 and fifth cycles are successively smaller, and placed at successively 

 longer distances from the mouth. Placed on either side of each tentacle 

 of the fifth cycle, and again somewhat nearer to the margin of the 

 calicle, are a pair of very small tentacles which have no septa developed 

 in correspondence with them. The number of tentacles is thus ninety- 

 six. The tentacles are light red in colour. Between their bases are 



