POLYZOA OR BRYOZOA. 425 



ally more or less branched, and consisting of small tubes 

 radiating from a central line, and opening at all points of 

 the surface, diminutive tubules sometimes occupying the 

 interspaces between the larger cells. The minute structure 

 of most of the forms generally placed under Ceriopora, as 

 well as of those named Trematopora and Helopora, is still 

 very little known ; but the Rhdbdomeson of the Carbonifer- 

 ous has been shown by Professor Young and Mr John 

 Young to have the curious feature that the cells spring from 

 a central hollow axis. It is possible that all of these, and 

 perhaps some other Palseozoic forms, may be ultimately 

 shown to be allied to the true Cerioporidce ; but their true 

 affinities and relations are at present more or less uncertain. 



We may here make a few remarks upon the position of 

 a number of common and widely distributed Palseozoic 

 fossils, belonging to such genera as Chcetetes, Monticulipora, 

 Callopora, Fistulipora, &c. These are now very generally 

 regarded as being really Polyzoa, and some of the forms 

 which have been described under some of the above generic 

 titles are, doubtless, really of this nature. Moreover, there 

 is a strong resemblance between certain of the above types 

 and some undoubted Tertiary and Eecent Polyzoa (e.g., 

 Heteropora). On the other hand, all the forms which can 

 be allowed to stand finally under any of the above heads, 

 have the feature that the tubes are crossed by transverse 

 calcareous partitions or " tabulce" these structures being 

 quite unknown among recent Polyzoans. They have, also, 

 strong and numerous points of agreement with various 

 forms (such as Alveolites, Favosites, Tetradium, &c.) which 

 cannot, at present, be removed from the Corals without 

 violence. It has therefore seemed best, in the meanwhile 

 at any rate, to consider the forms in question as being 

 Actinozoa, and as not certainly referable to the class now 

 under consideration. 



A fourth group of Palaeozoic Polyzoa includes a number of 

 forms which form thin crusts, growing parasitically upon 

 shells, corals, and other foreign bodies, and which may be 

 taken, in a general way, as ancient representatives of the 

 existing family of the Membraniporidce. The Silurian Pales- 



