48 SMITHSONIAN INSTITUTION 



to rather broad, in some cases appearing to ramify downward. Interbrachials 

 not protuberant. Plates low-convex, with crenulate margins, and radiating 

 ridges changing from fine to coarse upward, but with pits at the corners almost 

 everywhere sharp and specially conspicuous; those of the interbrachial pave- 

 ment often greatly modified by the sculpturing into strong bands, irregular 

 network, and stellate figures, in which the fixed pinnules can rarely be traced. 



Dimensions of specimens as usually found: Height to axillary IIBr, 

 40 to 60 mm. ; width at upper level, about the same ; an average well-preserved, 

 rotund specimen, slightly elongate in appearance, is 60 by 55 mm. 



I have proposed this species to receive a rather variable form which is 

 found in the Linden formation at several localities in Benton, Perry, and 

 Hardin counties, Tennessee. Only in the latter region have I been able to 

 obtain good specimens, but it must have been an abundant and well-distin- 

 guished species, to judge from the many fragments that have been seen in the 

 other regions mentioned. It is distinguished from .S\ elegans by the less 

 elongate form and fewer secundibrachs, and of course from its associated 

 species of the pratteni type by the general form. S. stellatus, if we knew it 

 from better specimens, might be found to include these forms. The pits at the 

 corners of the plates seem to be the most constantly conspicuous feature in the 

 sculpturing. In some specimens little else is seen ; in some the surface is dotted 

 more or less thickly with small knobs apparently formed by inorganic silicious 

 deposit. Chemical action has often greatly altered the superficial appearance, 

 producing a semblance of sculpture quite different from the normal. In some 

 cases the knobs lie chiefly upon the suture lines, exaggerating the connecting 

 ridges to such an extent that the plates appear to be surrounded by them. 

 In some the striae and crenulate margins are feebly shown ; while in others they 

 are strong and sharp, coalescing upward as usual. Erosion has sometimes 

 made the plates appear smooth and almost flat, especially in the lower parts. In 

 some specimens the ridge of secundibrachs is very wide, and in others rather 

 narrow; in some the basal part of the calyx is rather narrowly conical, and in 

 others somewhat obtusely and irregularly curved; rarely small median spines 

 are seen. Upon some of these differences my inclination was at first to pro- 

 pose other species, but there is nothing constant about them. In the inter- 

 brachial pavement most of the types common to 5". elegans are found among 

 these forms. 



The species is rather widely distributed ; in Hardin County, where the 

 best specimens were obtained, it is found chiefly in limestone layers at both the 

 Pyburn and Horse Creek localities, more or less irregularly throughout the 

 pyburnensis and pratteni beds. Occasional imperfect calices occur in Perry 

 County in the Decatur and Linden horizons, while in Benton County it is 

 numerously represented by weathered fragments from the large colony already 



