the Genus Tetradium, Dana. 167 



limestone pebbles are g'^Qjj pinkish, or mottled, composed 

 largely of comminuted organic remains, or coarsely oolitic. 

 The coral imder consideration presents itself under the form of 

 whitish or flesh-coloui'ed masses, an inch or more in diameter, 

 and of irregular shape. These masses break with a conchoidal 

 fracture, the texture being so extraordinarily dense and com- 

 pact that examination of unprepared specimens with a hand- 

 lens reveals absolutely no structure whatever, except that a 

 disposition of the mass in concentric layers may be seen, or 

 the aperture of some of the larger canals above spoken of may 

 be detected occasionally. Even polished specimens yield 

 little better results, and the aid of the microscope is necessary 

 to elucidate the unprecedentedly minute structure of the skele- 

 ton. The examination of thin transparent sections with the 

 microscope is also attended with unusual difficulties, to some 

 extent due to the infiltration of the delicate tissue with imper- 

 fectly translucent calcite, but also largely caused by the fact 

 that the hand-lens is unavailable in determining beforehand 

 in what directions to cut slices. Hence it is a mere matter 

 of chance whether sections in the desired planes can be ob- 

 tained ; and out of a large number of sections we have only 

 succeeded in preparing two which are in any portion strictly 

 at right angles to the long axes of the corallites. It need not, 

 therefore, be a matter of surprise if we are unable to speak 

 as to certain points connected with the structure of this coral 

 with all the precision that might be desirable. 



Before making a microscopical examination of this coral 

 we were disposed to think that it might turn out to be one of 

 tlie exceedingly dense Stromatoporoids, still undescribed, which 

 we have met with in the Upper Silurian of North America ; 

 but the investigation of thin sections leaves no doubt as to its 

 being a genuine coral. Nor can there be much doubt as to its 

 position ; for it must certainly be placed either in Tetradium 

 or in Chcetetes. The decision between tliese two genera rests 

 on the question as to the presence or absence of the septa 

 characteristic of the former ; and this question, owing to the 

 extreme tenuity of the corallites, their dense infiltration with 

 calcite, and the difficulty of obtaining sections accurately trans- 

 verse to the tubes, we have found a very hard one to determine 

 with certainty. Some of our transverse sections certainly do 

 not exhibit any septa ; but these are probably more or less 

 oblique, and under these circumstances the septa cannot be 

 detected even in tlic large and typical forms of Tetradium. 

 On the other hand, in two sections (figs, d, e) the corallites 

 not only show short and distinct septa, but also exhibit the 

 petaloid appearance so cliaracteristic of Tetradium. As this 



