126 REPORT 1871. 



so to the biologist, for these mixed and unnatural limitations and separations 

 only form gaps in his argument, which require bridging over. 



The Favositidae and Thecidae, Palaeozoic forms, may then be separated, for 

 the pui'poses of classification, from the Milleporidae and Seriatoporidae, which 

 are almost aU post-Palaeozoic ; but this hmitation is not to impede the plain 

 course of the palaeontologist, who studies from a biological point of view ; 

 nor is it to stand in the way of the assertion, that the break between the 

 Palaeozoic and younger Tabulata is almost nil. 



The genus Millepora belongs to the Milleporidae, and the ccenenchyma of its 

 species is very abundant. It is of " a very irregular and spongy structure, 

 rather than tubular " (Ed. & H.), The cahces are of very different dimen- 

 sions on the same coraUum. There are no distinct septa, nor is there a 

 columella. The tabulae are horizontal. These are the diagnostics of the 

 genus according to Milne-Edwards and Jules Hame. A careful examination 

 of the calices of good specimens determines that the trabeciilae, of which the 

 ccenenchyma is composed, often projects into them, in the position of septa; 

 but there is nothing like the regular arrangement as seen in Heliopora, or 

 in the Poritidae of the Perforata. The cells of the ccenenchyma may occa- 

 sionally be seen to open into the space above the last tabula. 



The absence of septa and this relation of the ccenenchyma to the gastric 

 space are most important. The tubular nature of much of the ccenenchyma 

 is evident, and longitudinal sections of some size prove that the spongy nature 

 of it is by no means constant nor uniform. 



In Heliopora, belonging also to the Milleporidae, the ccenenchyma is very 

 abundant, and covered here and there with rounded pores arranged more or 

 less regularly and separated by papillose granules. These grains are the 

 extremities of cylindrical "tigelles" which circumscribe the tubules, the 

 calice of which is open at the surface. The calices are circular. The septa 

 are slightly developed, and there are twelve of them. The tabulae are well 

 developed and horizontal (Ed. & H.). The nature of the ccenenchyma and 

 the distinct septa distinguish this genus from the last. Both of the extinct 

 species have a papillose or striated structure running over the coenenchymal 

 surface. In all the species the septa do not project far into the calice ; but 

 the amount of projection is not sufficient, as a structural peculiarity, in any 

 case to determine more than a specific distinction. Hence MM. Milne- 

 Edwards and Jules Haime when they separate, in their scheme of the Millepo- 

 ii^yd* , Millepora and Heliopora and other genera ivova. Hcliolites, Propora, 

 and Lyellia, the particular Palaeozoic genera, they can only be permitted to do 

 so on the plea that the plan renders the genera readily distinguishable. The 

 projection or non-projection is not sufficient to determine a generic difference. 



Now Heliolites has a beautiful ccenenchyma, very geometric, and not irre- 

 gular and spongy ; its cellules are placed regularly and symmetrically. In 

 most of the species the septa are distinct, and project far inwards, but in 

 Heliolites Orayi they are almost rudimentary, 

 ' The genus Pohjiremacis hnks Heliolites and Heliopora together, for its 

 ccenenchyma is that of the second, and the septa resemble those of the first- 

 named genus. Polytremacis is not older than Heliopora in the secondary 

 ages, and the septal distinction which cannot expel Heliolites Grayi from 

 its genus, and which is improperly allowed to distinguish Polytremacis and 

 Heliopora, and these and Heliolites, may well have been produced by varia- 

 tions in a succession of early secondary forms. 



» Op. cii. p. 225. 



