PAL^ONTOLOG V. 9 



diaphragms, and radiated by vertical crests, which in some forms 

 are very well developed, in others remain in rudimentary condi- 

 tion, or are entirely obsolete in individuals. Two principal groups 

 of tabulata are distinguished : 



I. Milleporidcs. — Compound polyparia built up by two structural 

 elements ; of larger radiated tubes, forming the visceral cavities for 

 the animal, and of a coenenchymatose tissue surrounding the tubes, 

 likewise either of tubular structure, or of a cellulose vesiculose 

 nature. 



II. Favositidce. — Compound polyparia formed of aggregated tu- 

 bules of equal structure, without intervention of any other tissue 

 element between the tube walls. The radiation of the tubes by 

 vertical crests is often more rudimentary in its development than 

 in the former order of Milleporidae. 



The palaeozoic forms of the Milleporidce are represented by the 

 genera : Heliolites, Lyellia, and Plasmopora, to which Milne-Edwards 

 adds Propora, Battersbyia, and Fistulipora. " The latter genus has 

 only an external resemblance to the Milleporidae. It belongs to the 

 Bryozoa, as I have already demonstrated in an essay published in 

 1866, in the proceedings of the Academy of Natural Sciences, of 

 Philadelphia, 



The genera Propora and Battersbyia I was not able to recognize 

 among the American fossils, unless a form which I have provision- 

 ally named Houghtonia should be identical with the Propora. 



The second order of Zoanth. tab., the Favositid(E, is grouped by 

 Milne-Edwards into six sub-orders : 



I. Favositinae. 2. Chaetetinae. 3. Halysitinae. 4. Pocilloporinae. 

 5. Seriatoporinae. 6. Thecideae. From these the Chaetetinae have 

 to be eliminated as a type belonging to the Bryozoa. 



The genera arranged under the Pocilloporinae, Seriatoporinae, and 

 Thecideae are in perfect conformity of structure with the Favosi- 

 tinae, and I see no reason for their separation from them — with one 

 exception, that of the genus Columnaria, which is a form of pecu- 

 liar type, exhibiting no great affinity with the genus Thecia, which 

 has been placed by Milne-Edwards in the same sub-order with it. 



The sub-order Halysitinae includes the genera Halysites, Syringo- 

 pora, Fletcheria, Thecostegites, and Chonostegites. The latter 

 two genera I place under the Favositinae as a form intimately 

 related in structure to Michelinia. The genus Thecostegites is only 



