GENUS POLYTREMA. 237 



nothing but a modified Rotaline, any person unacquainted with the essential characters of 

 this group might be excused for incredulity. But I cannot question that any candid 

 inquirer, who, having first mastered my general principles, may have followed me through 

 the preceding details, will be disposed to acquiesce even in this remarkable result. If it be 

 true that the character of the individual segments, as marked by the texture of the shell and 

 the mode of communication of the chambers, is a character of more importance than the 

 form which these segments may develope by their aggregation, then it cannot be questioned 

 that the serial assemblages of " globigerine" chambers {% 270) which form the turbinoid spire 

 of Discorbina, the outspread disk of Planorbulina, the conoidal or spheroidal mass of Tino- 

 porus, and the arborescent ramifications of Polyirema, are all most intimately related one to 

 the other. In the general plan of its chambered structure, Poli/frema is obviously most 

 nearly allied to Tinojiorm ; but if I am correct in the idea I have formed of its early mode of 

 growth, its fundamental affinity is rather to Planorbulina, since its subsequent increase takes 

 place, as in. that genus, upon only one side of the original spire, instead of upon both sides as 

 in Tinoporus. It is not a little curious that there should be a strong external resemblance 

 between Poh/trema and some of the less regular forms of Carpenteria ; a resemblance which is 

 increased by the presence of free openings at the extremities of the branches in the former, 

 and by the precise conformity which its areolation often presents to that of Carpenteria. The 

 relation is one, however, of mere isomorphism, as we have seen the internal structure of the 

 two organisms to be essentially different. 



412. Geographical Distribution. — All the specimens of this genus hitherto collected have 

 been obtained from the surfaces of shells, zoophytes, stones, &c., brought from tropical or 

 sub-tropical regions ; the large foliated bivalves of the Indian and Polynesian seas being its 

 favorite habitat. — We have not as yet any certain knowledge of the occurrence of this type 

 of structure in a fossil state ; but it is very probable that a considerable number of the arbo- 

 rescent fabrics which have been detached by D'Orbigny from the genus Ceriopora and thrown 

 into Polytrema will prove on examination to be referable to it, and will have to be removed 

 from the Bryozoa, with which they are at present associated. 



