234 



HYDROZOA. 



or vesicles, which are arranged in concentric layers round a large 

 axial tube running the whole length of the organism. This axial 

 tube is intersected by strongly curved calcareous partitions or 

 " tabulae " ; and the vesicular tissue which surrounds it is sometimes 

 found to be traversed by columns which radiate outwards to the cir- 

 cumference, and which may be compared with the " radial pillars " 

 of Labechia. Upon the whole, with our present knowledge, it would 



c \ 



Fig. 118.- — A, A fragment of Beatricca undulata, slightly less than the natural size, from the 

 Ordovician rocks (Cincinnati Group) of North America, b, Cross-section, and c, Vertical section 

 of Beatricca nodulosa, from the same formation, slightly enlarged. The sections show the 

 peripheral vesicular tissue and the central tabulate tube. (Original.) 



seem best to regard Beatricea as a very abnormal type of the Stro- 

 matoporoids, with relationships to Labechia and Idiostroma ; but the 

 position of this anomalous genus must be regarded as more or less 

 doubtful. 



The remaining forms of the Stromatoporoids present certain strong 

 resemblances in their structure to the existing genus Millepora, and 



