6o THE GENUS MONTICULIPORA. 



not affect the fact that Dr Lindstrom has not observed — so far 

 as I am able to understand his very clear account — the actual 

 transition between an undoubted encrusting Ceramopora and an 

 undoubted free and discoidal specimen of Monticnlipora petro- 

 politana, Pand. He has examined certain specimens which 

 show characters linking the one on to the other ; but I do not 

 understand him to assert that he has examined specimens 

 which in one portion show the unmistakable characters of Cer- 

 amopora, and which in another, demonstrably older portion, 

 exhibit the features proper to Monticnlipoi^a. I cannot, how- 

 ever, accept any specimens except such as exhibit as individ- 

 uals the characters of the two types, as being proof that either 

 of the types in question has been developed out of the other. 

 In the second place, apart from this general argument, which 

 may easily be pushed too far, there are very strong grounds 

 for regarding Ceramopora as an independent organism quite 

 distinct from all the forms of Monticiilipora. Thus Ceramo- 

 pora is most abundant in Upper Silurian and Devonian strata, 

 in which MonticnliporcE are comparatively rare fossils, while the 

 genus is but poorly represented in Lower Silurian strata (such 

 as the Cincinnati formation in North America), in which Monti- 

 culiporce are excessively abundant. An additional proof of the 

 distinctness of Ceramopora is found in the fact that it grows to 

 a large size, preserving unchanged its normal and proper char- 

 acters, while the general structure and form of its tubes are 

 markedly unlike those of the corallites of the MonticitliporcE, 

 being reclined, with oblique and often crescentic mouths, and 

 being either devoid of tabular, or possessing but a small num- 

 ber of these structures. (A few tabulae are present in Ceramo- 

 pora 0/iiocnsis, Nich., and they exist in larger numbers in what 

 is probably an undescribed species of the genus from the 

 Wenlock Limestone of Dudley ; but I have not detected them 

 in the more typical C. Httronensis, Nich,, and in similar thin 

 encrusting forms.) Moreover, the colonies of Ce^-ainopora are 

 usually (always ?) fixed, being attached parasitically by a por- 

 tion or the whole of the lower surface to some foreign body ; 



