J". M. Safford on the Genus Tetradium. 



287 



die of each side externally, opposite the lamellae. L 

 Figure 1 will serve to give an idea of the trans- pn 

 verse, or horizontal section of one of these tubes. CO 

 In the massive specimens the horizontal sections tnb? a J? tlftSJJSI 

 of the tubes are square, or nearly so. In all of H ans Y£™P Q s ^ tiop » 



. ii i magnified 3 or 4 tunes 



the species the walls are more or less rugose. Hne»r. 



The increase appears to be by the division of the tubes, the 

 latter splitting sometimes into two cell-tubes, not unfrequently 

 perhaps into four ; opposite laminae unite and form the new 

 walls of the young cells, each of which is in the mean time sup- 

 plied with its four rays. 



Among the numerous specimens of this genus, which we have 

 seen, we have met with but one which shows clearly the pres- 

 ence of transverse septa. This is a fragmentary specimen of the 

 first species described below. In it the septa are distant about 

 twice the breadth of a tube ; but few however are seen, and these 

 are confined to one end of the mass. 



This group we regard as being allied in some respects to the 

 Favositidce, while on the other hand, the cruciform arrangement 

 of the lamellae unite with the Zoa'nthana rugosa of MM. Milne 

 Edwards and Haime; in fact it appears to afford an interesting 

 type of the quadripartite character of the lamellae, first pointed 

 out, by these distinguished authors, in many palaeozoic corals. 



We enumerate the following species, all of which as well as 

 the genus itself, so far as we know, are confined to the Lower 

 Silurian rocks. 



1. Tetradium fihratum Safford, (Fig. 2.) — Cor- 2. 

 alia massive, hemispherical, or flattened hemis- 

 pherical, composed of diverging tubes. Cell- 

 tubes four-sided with thin and slightly rugose 

 walls ; the four lamellae distinct, nearly reaching 

 the centre of the tubes ; breadth of full-grown 

 tubes usually about, or but little more than, 

 half a line, varying occasionally from id to jths m 



n ' J ° J n 1 Transverse section 



01 a line. lransverse septa usually absent, of a few tubes of t. 

 A few have been seen in one specimen, which fibialmilj ma s ni ned. 

 were about twice the breadth of a tube apart. 



This beautiful species, which may be taken as the type of the 

 genus, occurs abundantly throughout the upper half of the Lower 

 Silurian rocks of Middle Tennessee, associated with FaviMla 

 stellata Hall, Ambonychia radiata Hall, and other Hudson Kiver 

 species. Large masses a foot or two in diameter, are met with. 

 The calcareous specimens often resemble, in a weathered longi- 

 tudinal section, a fossilized but previously somewhat macerated 

 mass of woody fibre, and hence the name of the species. 



2. T. columnare Hall ; Syn. Chcetetes column aris Hall. Pah 

 of K Y., vol. i, p. 68, PI. xxiii, Figs. 4, 4 a — Mr. Hall's species, 



