138 PBOF. p. MARTIN DUKCAN'S REVISION OE" THE 



The subgenera are not very satisfactory, on account of the 

 influence of fossilization and wear and tear in destroying the colu- 

 mella or in modifying it. But if it can be established that a 

 species has had no columella, or has a papillary one, or one which 

 i s essential and styliform or knobbed, it is right to put it in the 

 characteristic subgenus. Thus the true Thamnastrseans, accord- 

 ing to Milne-Edwards and Jules Haime, have a papillary colu- 

 mella (Hist. Nat. des Corall. vol. ii. p. 555). The Thamnastraese 

 with a knob or styliform process for columellse belong to the sub- 

 genus Oentrastrcea, d'Orbigny. In most cases sections show the 

 relics of the columella. 



Mr. E,. Tomes, E.Gr.S., follows M. de Fromentel in his classifi- 

 cation (Quart. Journ. Greol. Soc. 1882, p. 434), and gives him the 

 authorship of Synastrcea as well as Gentrastrcea, the first being 

 a creation of Milne-Edwards and Jules Haime, and the last of 

 d'Orbigny ! As the subject has been now thoroughly discussed, it 

 is only necessary to revise the genus TJiamnastrcea after Milne- 

 Edwards and J. Haime, and to admit, for the sake of convenience, 

 Gentrastrcea as a subgenus. Mr. E. Tomes suggests the term 

 septal-costce for the costae which join the septa of different calices 

 in TJiamnastrcea. The term septo-costce was used half a century 

 ago, and has been employed ever since, and is the better term of 

 the two. 



Grenus Tham-ntastrtra, Lesauvage, amended; post. Ed. Sf S. (L857), 

 Hist. Nat. des Gorall. vol. ii. p. 555. 



Syn. Synastrcea. 



Colony massive, convex, subplane, or gibbous, or thin and 

 expanding, or in layers, pedunculate or with a broad base, or in- 

 crusting, or more or less dendroid in shape. Corallites indi- 

 stinctly defined. They are united by costse or by an ill-developed 

 wall. Calices superficial, centres distinct, and the intercalicular 

 space variable in extent. Columella variable, papillary, nodular, 

 small, styloid. Septa merging into septo-costge which are confluent 

 with those of neighbouring calices, usually flat on the free border, 

 dentate, the minute projections being the tops of lines of trabe- 

 culae, stout or thin, solid or perforate here and there ; straight, 

 wavy, curved, or geniculate, long or short, often uniting or not. 

 Dissepiments rudimentary or decidedly developed. Synapticula 

 numerous, small. Glemmation submarginal. Common colonial 

 wall, when it exists, costulate, epithecate, and with synapticula. 



