GENERA OE CH.ETETID.E AND MONTI CULlFORIDAi. 261 



described by Mr Lonsdale (Geol. of Riiss., vol. i. p. 595, 1845), 

 who drew special attention to the fact that the walls of the 

 corallites are inseparably united, so that fractures expose the 

 iiitcrioj' of the tubes, this structure depending upon the fissi- 

 parous mode of increase of the coral. M'Coy (Brit. Pal. Foss., 

 p. 82, 185 1) may be considered as entirely accepting Mr Lons- 

 dale's views as to the characters of the genus Chcetctcs. Milne- 

 Edwards and Haime (Brit. Foss. Cor. Intr., p. 61, 1850), while 

 accepting the genus, ignore the feature just alluded to as so 

 strongly emphasised by Mr Lonsdale, and add no character 

 which could be accepted as in any way of generic value. In 

 the " Polypiers Fossiles" (p. 261, 185 1) the same authors give 

 a fuller account of Chcstetes, and they now unite with it the 

 genus Steiiopora, Lonsd., and also the ill-characterised type 

 which D'Orbigny had named Monticulipo7''a (Prodr. de Paleont. , 

 t. i. p. 25, 1850). At a still later period (Brit. Foss. Cor., p. 

 264, 1854), the two distinguished French observers so far 

 altered their views that they accepted Monticulipora, D'Orb., 

 as distinct from Cha^tdcs, Fischer, the ground of distinction 

 being that in the former the corallum increases by gemmation, 

 whereas in the latter the mode of growth is by fission. Most 

 subsequent writers have followed the course ultimately adopted 

 by Milne-Edwards and Haime, so far as concerns the generic 

 distinctness of Chcetetes and Monticidipoi-a, and the grounds 

 of this distinction. In a paper, however, upon the species of 

 Chcetetes in the Lower Silurian rocks of North America 

 (Quart. Journ. Geol. Soc, vol. xxx. p. 499, 1874), I formerly 

 expressed the opinion that the genera Chcrtctes and Monticuli- 

 pora were not sufficiently differentiated, and that the mere 

 mode of growth, even if admitted to be of generic value, was a 

 character so difficult, in many instances, of determination, that 

 it should not be regarded as of itself sufficient to separate two 

 types otherwise closely allied. At the same time I stated that 

 I thought Stetwp07^a^ Lonsdale, to be insufficiently character- 

 ised, and pointed out that different observers had defined this 

 genus by means of very different and in some cases compara- 



