322 TABULATE CORALS. 



Trenton Limestone species. On the other hand, a minute 

 examination of the corals of the Hudson River group of 

 Canada, which have commonly been spoken of as " puff-ball 

 varieties of Stenopora fibrosa" and which I used to regard (op. 

 jam cit. p. 176) as a mere variety of M. petropolitana, Pand., 

 has shown me that these are in reality entirely identical in 

 internal structure with the M. undulatd of the Trenton Lime- 

 stone, from which they only differ in their smaller size and 

 hemispherical or spheroidal shape (PI. XIV., fig. 3). I have 

 figured thin sections of both for comparison. 



Tangential sections of both the Trenton Limestone and 

 Hudson River group examples of M. undulata (PI. XIV., fig. 

 4 and fig. 3 a) show the corallites to be strikingly thin-walled, 

 and markedly angular, while, except for the occasional presence 

 of a cluster of somewhat extra-sized tubes, their dimensions are 

 very uniform. Here and there, of course, small corallites will 

 occur, but these are simply young tubes, such as would be seen 

 in any tangential section of a Favosites, and they do not form 

 part of a regular series of special corallites. That this view is 

 correct is shown by their very occasional occurrence, but is still 

 more conclusively proved by vertical sections (PI. XIV., figs. 

 3 b and 4 a). These show that all the corallites those forming 

 the clusters as well as those composing the mass of the colony 

 are precisely similar in their structure, and are not divisible 

 into a series with remote and one with crowded tabulae. All 

 alike have thin, flexuous, often closely undulated walls, and in 

 all alike the tabulae are delicate horizontal plates, situated at 

 distances of from a quarter of a line to nearly a line. In all 

 the specimens I have examined there is, also, an evident 

 periodicity of growth, tabulae being periodically developed at 

 corresponding levels in all the tubes, so that the entire corallum 

 breaks up into concentric layers. 



Formation and Locality. Rare in the Trenton Limestone of 

 Peterboro', Ontario. Common (the " puff-ball variety ") in the 

 Hudson River group of Toronto, Weston, and other localities 

 in Ontario. 



