HICKSON— POLYTREMA 451 



name of the Mediterranean form must be Polytrema miniaceum, it is also necessary to 

 retain the specific name cylindricum for the type specimens of Sporadotrema from the 

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

 The specific name of the only species of the genus Homotrema should be "rubrum" 

 (Lamarck). 



But when it comes to a question of subdividing the three genera into a number 

 of specific groups a much more debateable ground is reached. 



The three genera described in this paper are sedentary animals and have, in all 

 probability, a very wide geographical distribution. Beginning their lives as free floating 

 organisms they settle down casually on some hard substance at the bottom of the sea 

 and gradually assume a coralline form. The actual environmental conditions to which 

 the individuals are subject during the remainder of their lives vary enormously according 

 to the locality, depth, condition of the tides and the neighbouring benthos of the spot 

 on which they have happened to settle, and these varying conditions undoubtedly exercise 

 a considerable influence in the determination of the actual size, shape and ramification 

 of the full grown individual. It cannot be assumed that the form of a full grown 

 individual is entirely a matter of environmental influence. A comparison of a large 

 number of specimens of Homotrema and Polytrema must convince any one that there 

 is an underlying hereditary influence controlling to some extent the growth of the 

 individual. It is impossible to determine in any particular example, with any pretence 

 to scientific accuracy, the part played by the environment and the part played by 

 heredity in the determination of shape, and it becomes therefore a matter of judgment, 

 based upon a knowledge of a large number of specimens of each genus, whether a 

 particular specimen or group of specimens should be separated from the others as a 

 distinct species. 



In many cases among sedentary animals, a particular form of growth seems to be 

 associated with some particular conditions of the environment such as we see for example 

 in many forms of corals such as Millepora and Madrepora, and the specimens may be 

 arranged in groups, which are practically discontinuous groups, according to their shape. 

 Detailed examination of the structure of the members of these groups may show no 

 differences of importance correlated with the differences in shape, and it appears to 

 me that to regard such groups as separate species is unsound. Nevertheless it is of 

 importance to be able to give to them group names, in order to be able to record 

 briefly the general influence of the environment on the specimens from a particular 

 locality or depth, and such group names I have in a previous paper (8) called "facies." 

 As regards the two genera Polytrema and Homotrema I do not consider that we are 

 in a position to subdivide the two species. I believe that a more detailed comparison 

 of the structure of the S. Pacific, Indian Ocean and Mediterranean forms of Polytrema 

 may necessitate the constitution of two or three new species. There are some differences 

 to be observed between the Polytremas from the Mediterranean and those from the Indian 

 Ocean, but I am not in a position to state that these differences are constant. 



For the present I consider that there is only one species of Polytrema (P. miniaceum 

 Pallas) and it is convenient to recognise in this species two "facies," the flat plate like 



