ON FOSSIL POLYZOA. 173 



raents of this genas, which will enable me to carry back the type to 

 Silurian times. Mr. R. Tate has already carried back the genus to 

 the Lias ' but the specific difi'erences between the Liassic and Silurian 

 forms are very marked. The Silurian species I shall describe under the 

 name of Spiropora regularis, Vine. 



1874. Botnjllopora, Nicholson.^ 



This curious genus, founded by N'icholson for Devonian species, is 

 allied to Defrancia and Lichenopora, but unlike either. The author says 

 ' I have been unable to refer these singular Polyzoa to any existing group, 

 and have therefore been compelled to found a new genus for their recep- 

 tion. Zoarium calcareous, sessile, and encrusting, forming systems of 

 small circular discs, the upper surfaces of which ai^e marked with radiat- 

 ino- ridges, upon which the cells are carried. Each disc is attached by its 

 entire lower surface, slightly convex above, with a central nonporiferous 

 -space, round which a number of radiating poriferous ridgep occupy an 

 exterior, slightly elevated zone. Cells forming a double series on each 

 -ridge, immersed with rounded mouths,, which. are not elevated in any 

 part of their circumference above the general surface.' ^ 



One species is given, B. socialis, Nich. PI. ix. fig. 10, and it is not of 

 very rare occurrence in the Hamilton Formation. I have not seen among 

 any of our own Palceozoic Polyzoa any approach to this genus. It may 

 be well to direct attention to the characters, because workers may find 

 even this amongst the group of our hitherto most neglected fossils. 



In my first Report (' British Carboniferous Polyzoa,' 1880^) 1 said that 

 " to the Palteontologist the study of the Palteozoic Polyzoa opens up many 

 very important biological details; for the connection of the Polyzoa Avith 

 the Graptolites is a question that must be dealt with in detail." 



Since this was written I have gone over much that has been written 

 in this country on this debatable subject. Professor Huxley, Mr. Salter, 

 and Professor H. Alleyne Nicholson have severally occupied themselves 

 .with this question of affinity. Mr. Salter says, ' I think Professor Huxley 

 'first suggested the resemblance to Defrancia ' ^ ; his own opinion, how- 

 ever, was very decidedly expressed. ' The point I would chiefly call 

 attention to is that there is a complete series up to the most compound 

 in this remarkable family ' ; and after pointing out the varied features of 

 the leading types of the GraptoUtidce, he concludes by saying ' Dendrograptus 

 has the branches numerous, nnsymmetrical, and crowded, while Didyonema 

 completes the series by showing the numerous rod-like stems each with 

 their cells in double rows, connected by numerous transverse bars into a 

 network like that of Fenestella, to which, indeed, I believe it forms the 

 passage group.''' Professor Nicholson, after examining in detail the 

 various points raised by Mr. Salter, says, ' The " polyzoarium " (of the 

 Polyzoa) is commonly more or less highly charged with lime, and this is 

 especially the case with the fossil-forms. The polypary of the Graptolites, 

 on the other hand, are invariably corneous (or chitinous).' ^ Notwithstand- 

 ing these varied opinions, I very reluctantly reviewed the whole of the 

 points mooted by Nicholson and others, and then submitted my notes to 



' Spiropora liassica, Tate, Geo. Mag., 1875. 



2 Canadian Jour., No. 80, Geo. Mag., 1874, p. 23. ^ Ibid. p. 23. 



■• JBritixh Association Reports. 



5 Memoirs oft/ie Geohgical Survey — North Wales, p. 328, 1866. 



6 Ibid. ' British Graptnlitidce, p. 85. 



