HYDROCORALLIXES AND STROMATOPOROIDS. 



5 



Eocene rocks ; and in the same formation appears the allied genus 

 Axopora, which differs from Millepora chiefly in the fact that the 

 tabulate zooidal tubes are traversed by a large fasciculate " colum- 

 ella ? ' or central rod. The Cretaceous genus Porosphcera has been 

 regarded as related to Millepora, but it appears to be truly referable 

 to the Sponges. 



The second group of the Hydrocorallines is that of the Stylaster- 

 idce, comprising a number of recent genera (Stylaste?-, Allopora, 

 Cryptohelia, Sporadopora, <xx.), all of which are inhabitants of the 

 sea, and range from the neighbourhood of the coast-line to great 

 depths. The ccenosteum of the Stylasterids is calcareous, more or 





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d° 



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Fig. 109. — Thin sections of Millepora, sp., enlarged about thirty-five times, a, Tangential 

 section, b, Vertical section, g g, Gastropores ; d d, Dactylopores ; c c, Ccenosarcal canals. 

 (Original.) 



less branched, forming a dendroid colony or flabellate expansion, 

 and exhibiting upon the surface, or on its sides, small rounded aper- 

 tures which usually have the appearance of being intersected margin - 

 ally by radiating partitions or septa, and thus simulate the " calices " 

 of an ordinary Madreporarian Coral (fig. 1 1 o). In other cases, the 

 surface shows a series of large apertures, with more numerous and 

 irregularly arranged smaller openings, the latter not being radially 

 arranged round the former. Thin sections of the ccenosteum (fi^. 

 in) show that the general skeleton is composed of a dense, sub- 

 crystalline calcareous tissue, which is traversed in all directions by a 

 system of branched and anastomosing canals, which in the living 

 vol. 1. p 



