HICKSON— POLYTREMA 447 



some white specimens, but it is probable that all of these were technically dead. The 

 distribution of Homotrema is still imperfectly known, but I have seen specimens from the 

 Ki Islands 129 fms. and from Celebes in the Malay Archipelago, from various localities in 

 the Indian Ocean, from the West Indies, and from the Coast of S. America. 



In both Homotrema and Polytrema, as in other sedentary coralline structures, the 

 form of the full grown skeleton is very variable, but the study of a large number of 

 specimens shows that in Polytrema the ramification is more complete and the branches 

 longer and more slender than in Homotrema. This difference between the genera is not 

 only indicated by Pallas (p. 446 footnote), but also by Lamarck, who by the use of the word 

 " sublobata " instead of " subramosa " suggests that his own specimens of Millepora rubra 

 did not branch. In Homotrema the larger specimens often assume a mushroom shape, the 

 free end being considerably expanded and giving off short blunt processes, whereas the 

 proximal end is contracted into a relatively slender stalk. 



The combination of the two characters of colour and form are frequently sufficient to 

 determine a specimen without the use of any magnifying glass at all. But of course no 

 specimen can be determined with certainty to be a Polytrema or a Homotrema until its 

 surface characters have been examined by at least a half-inch microscope objective. 



The description of the structure of Homotrema for comparison with that given for 

 Polytrema on p. 44 o is as follows. 



The surface is marked by clearly defined areolae about O'l mm. in diameter 

 perforated by a large number of small foramina, '001 mm. in diameter. The boun- 

 daries of the areolae are solid, and there are no pillar pores. Below the surface 

 there may be seen a number of chambers communicating with one another by 

 large open passages and bounded by solid walls. There are no hollow pillars 

 and no foramina except those on the outer walls of the superficial chambers 

 (Plate 32, figs. 19, 22, 28). 



3. Sporadotrema. 



The specimens of this genus that were first discovered were found by Captain Warren 

 in the Gulf of Manaar and described by Carter (5) under the name Polytrema cylindricum. 

 By the kindness of the authorities of the Public Museum at Liverpool I have been able to 

 examine the type and co-type specimens. They belong to the " Amirante " facies* of 

 Sporadotrema but are very small specimens. The type specimen is 6 mm. in height and 

 2 mm. in diameter. The illustration of Polytrema miniaceum in Brady's Plate CI, fig. 5, 

 of the Challenger " Foran.inifera " represents a Sporadotrema. There is a specimen of 

 Sporadotrema similar in general structure to the " Amirante" facies in the British Museum 

 from the Macclesfield Bank and there is also in the same institution a pink specimen 

 resembling the " Saya de Malha " facies. I have examined the type specimen of Carter's 

 Polytrema mesentericum in the British Museum and found that it belongs also to the 

 genus Sporadotrema, but it is a distinct species. The type specimen is very much water- 

 worn and the locality from which it came is unknown, hut I have found a number of fine 

 specimens in a collection of Alcyonaria made by Professor Haddon in Torres Straits. 



* For an explanation of the use of this term see p. 451. 



57— -J 



