the Genus Tetradium, Dana. 163 



tubes, which diverge from the base or from an imaginary 

 axis, and which are not arranged in superimposed layers. 

 Increase by tission of the old tubes. Corallites of great length 

 (sometimes three inches or more), generally about a third of a 

 line in diameter, their shape usually irregularly four- or five- 

 sided. Septa varying in number from one to four, most 

 commonly the latter, rarely five. Tabulae complete, mostly 

 about three in the space of 1 line. Walls thick, imper- 

 forate. 



fe;:^,'i^ \'^}m 



' y 



a, a small portion of a transverse section of Tetradium minus, mag- 

 nified about ten diameters. The section is a transparent one ; and 

 the visceral chambers of the corallites, being filled up -with the 

 matrix, appear black, whilst the walls are composed of crystalline 

 calcite; in the centre of the section is a vacant space filled with 

 calcite (a water-canal ?). b, portion of polished transverse section of 

 same, not so highly magnified ; in this section the visceral chambers 

 appear as light spaces and the walls are dark, c, portion of a longi- 

 tudinal section of the same, showing tabulse. d, portion of a trans- 

 parent transverse section of Tetradittm Peachii, Nich. and Eth., jun., 

 magnified about twenty diameters. Near the bottom of the .section 

 a water-canal (?) is seen cut ac-ross. c, portion of transverse section 

 of the same, magnified still further. /, portion of the longitudinal 

 section of the same, magnified about fifteen diameters, g, portion of 

 the same, enlarged stiU further. 



Ohs. The large and massive corallum of this species, at first 

 sight, recalls to mind the most bulky of the species of Chcetetes^ 

 from which, however, it is at once distinguished by the posses- 

 sion of distinct septa. These structures are sometimes difficult 

 to make out in portions of the corallum, especially in simply 

 weathered surfaces ; nor can they be readily detected in oblique 

 sections of the corallites. They seem, indeed, to have been 

 very readily broken away ; and as the outer portions of the 



11* 



