HICKSON— POLYTREMA 445 



The coral may be in the form of small flat encrusting disks on corals, shells and rocks, 

 or in the form of short branching coralline structures rising from a flat and sometimes 

 spreading base. The usual size of the branching forms from the Mediterranean Sea is 

 about 3 — 4 mm. in height and 4 — 5 mm. in diameter at the base. The flat encrusting 

 forms are very variable in shape and size, the largest patch from the Mediterranean 

 1 have seen is 6 mm. in length but of irregular shape. The specimens from other parts 

 of the world do not seem to grow much larger than the Mediterranean specimens. The 

 greatest height attained by the branched specimens in Mobius' large collection off' Mauritius 

 was only 3 - 5 mm. The finest specimen I have seen came from the Ki Islands in the Malay 

 Archipelago (Plate 30, fig. 1, Plate 31, fig. 8), Siboga station 250, 90 metres. It is 7 mm. 

 in height and one branch alone is 6 mm. in length. 



The colour is usually pale red, of a tint that has variously been described as 

 "cinnabar," "carmine" or "peach colour," but pale pink and white varieties are known. 



There can be no doubt that the genus has a very wide distribution in the tropical and 

 temperate seas. It certainly occurs in the Mediterranean Sea, the Indian Ocean, the Malay 

 Archipelago and the S. Pacific Ocean (Kermadecs, Funafuti, etc.) but I have not yet found 

 any specimens from the W. Indies or from the shores of the American continent. 



It is not necessary to describe the structure of Polytrema in detail but it may be 

 useful for purposes of comparison with the two allied genera (cf. pp. 447 and 450) to state 

 the principal characters that can be used for distinguishing the genus. 



The surface is perforated by two kinds of pores, the larger or pillar pores 

 ("Pfeilerporen" of Merkel) gradually shelving from a diameter of 0"08 mm. to 

 a diameter of 0"03 mm. and the smaller and far more numerous pores or foramina 

 opening abruptly with a diameter of about 0"005 mm. (Plate 32, fig. 23). Below 

 the surface there is a series of laminae perforated by foramina similar to those 

 of the surface lamina and these laminae are connected together and supported 

 by the hollow pillars (Pfeiler) (Plate 32, figs. 18 and 27). The walls of the 

 pillars are not perforated by foramina but there may be one or more than 

 one passage ("Lochern" of Merkel) by which the cavities of the pillars are 

 connected with the intralaminar spaces. 



2. HOMOTREMA. 



It is difficult to determine whether the original specimens described under the names 

 Millepora miniaeeum, Polytrema corallina and Polytrema miniaceum by Pallas, Risso 

 %and Dujardin belong to this new genus Homotrema or to the genus Polytrema. Lamarck 

 (10) described his specimens of Millepora rubra, some of which came from the "American 

 ocean," as " sublobata, poris crebris minutis punctata." I believe these specimens belong 

 to the new genus Homotrema. The specimens described by Max Schultze (15) clearly 

 belong to the genus Polytrema and it is evident that the specimens described by Carpenter 

 (2) belong to the genus Homotrema. Max Schultze 's specimens came from the Mediterranean 

 and he states that he examined Carpenter's specimens which came from the South Sea, and 

 although they showed more variation than his, he had no doubt they were the same specific 

 form. Schultze's opinion that the specimens from the two localities are undoubtedly the 

 SECOND SERIES— ZOOLOGY, VOL. XIV. 57 



