238 



J. M. Safford on the Genus Tetradium. 



we think referable to this genus. It differs from T. fibratum in 

 the following particulars : the tubes are not as uniformly four- 

 sided, nor are they arranged with equal regularity ; the walls are 

 more strongly rugose ; the lamellae appear to have been more 

 delicate, and are generally not to be seen ; traces of them how- 

 ever can, in most instances, be found upon close examination. 

 The four-sided character of the tubes is sufficiently well marked 

 to justify this reference, in connection with the fact that traces of 

 the lamellae can often be detected. 



This species is associated with the last, and occurs, in addition, 

 lower in the series, with Columnaria alveolata Hall. It is a 

 common fossil in our Central Basin. 



S. T. apertum Safford — Tubes isolated or fasciculated, or else 

 united in linear series which often intersect, forming irregular 

 reticulations ; breadth of tubes about half a line ; lamellae as in 

 T. fibratum. 



This species includes certain open, loosely constructed corals 

 which belong to this genus. Two varieties may be designated. 

 These appear to run into each other in some specimens, though 

 it may be found necessary hereafter to separate them. 



(a) Masses composed of separate tubes occasionally united by 

 their sides. These forms often resemble Syringopora. 



(b) Masses composed of tubes arranged in linear series, the lat- 

 ter intersecting and forming masses like those of Halysites eaten- 

 ulatus Linn. 



Should it be found necessary to separate these varieties, the 

 first may be designated T. laxum and the second T. apertum. 



We have observed no characters, with the exception of the 

 open mode of growth which separate this species from T. fibra- 

 tum. 



The first variety is abundant in the middle part of the Lower 

 Silurian series of Middle Tennessee. The second is found in the 

 upper half as well as near the base. We have observed the 

 same species in Kentucky. 



4. T. minus Safford — We include in this species massive speci- 

 mens, (generally small,) the tubes of which are only from -Jth to 

 •J-d of a line in breadth. The tubes in some specimens are quite 

 regular, in others, though generally four-sided, are more or less 

 irregular and have the aspect on the upper surface of Chaetetes. 

 Lamellae as in T. fibratum. 



We have occasionally seen this species in the upper division of 

 the Lower Silurian series in Middle Tennessee, as well as in 

 Kentucky. 



