100 Proceedings of the Eoyal Physical Society. 



cannot enter here into the structure and affinities of this 

 group, but I may say that I do not think there is sufficient 

 evidence for the union of the Auloporidce with the Syringo- 

 poridce, as supposed by some palaeontologists ; and I think it 

 not unlikely that the former will prove eventually to be re- 

 ferable to the Alcyonarian division of the Actinozoa. Clado- 

 chonus, M'Coy (= Pyrgia, Edw. and H.), seems to be insepa- 

 rable from Aulopora; but the Carboniferous G. crassus, 

 M'Coy, is a special generic type, destitute of all affinity with 

 the group with which it has been hitherto associated. 



VII. Halysitid^. — This group is typified by the common 

 chain-corals (Ilalysites) of the Silurian rocks. In this genus 

 the corallum is composed of long cylindrical corallites, united 

 to one another by their sides only, along their whole length, 

 but otherwise free. Between each pair of corallites — typi- 

 cally, but not invariably — is situated a single much smaller 

 tube, which is destitute of septa, and possesses numerous 

 close-set tabulae. The larger tubes have complete, com- 

 paratively remote " tabulae," and often have minute spiniform 

 septa. The walls of all the tubes are imperforate, and neither 

 mural pores nor hollow connecting-processes are present. 



The family of the Halysitidm was made by Milne-Edwards 

 and Haime to include Syringopora and its allies; but the 

 fact that the visceral chambers of contiguous corallites com- 

 municate in the latter directly by means of hoUow connecting- 

 processes, whereas the tubes of Haly sites are enclosed by 

 imperforate walls, sufficiently proves the distinctness of these 

 two types. Upon the whole, I am inclined to think that 

 Halysites will find its nearest ally among the Helioporidcc, 

 which it resembles in one important particular, namely, in the 

 general possession of two sets of tubes, indicating the exist- 

 ence of two sets of zooids. Moreover, the tubes of the one 

 series are of large size, with remote tabulae and with radiating 

 septa ; while those of the other series are small, have close- 

 set or vesicular tabulae, and are destitute of septa. If this 

 conjecture be correct, then Halysites must be removed to the 

 Alcyonaria ; but it will in any case form the type of a dis- 

 tinct group. 



VIII. TETKADiiDiE. — This group includes only the singular 



