68 LOWER PENINSULA. 



THECIA MINOR, N. Sp. 



[Vide Roemer, Silur. Fauna of Tennessee, Tb. 2, Fig. 4.) 



TlIECIA SWINDERIANA. 



General structure perfectly conformable with the former species. 

 Discoid expansions covered on the lower side by an epitheca, con- 

 centrically wrinkled, and exhibiting the prostrate tube channels 

 diverging from the centre. The main difference between the two 

 forms lies only in the size of their tube's, which in this latter form is 

 only one millimeter instead of two, as in the former. The speci- 

 mens vary considerably in their surface characters. Some have 

 only moderately stout tube walls, and join with gently dilating 

 mouths under edged polygonal margins. With the increase of the 

 thickness of the walls the orifices lose their circumscribed form, and 

 the thickened walls combine into a common, broad, interstitial sur- 

 face impressed with small, circular, radiated cell pits. The radial 

 crests often extend across the interstitial spaces from one orifice 

 into the other as superficial rugae, mingling with additional irregu- 

 lar rugae and granulations by which the interstitial surface is deco- 

 rated. By a still greater degree of incrassation of the tube walls 

 the orifices become almost closed, punctiform, while the lateral pore 

 channels remain as large as ever, and prolongate in proportion to 

 the thickening of the wall substance. Such specimens appear as 

 massive expansions, perforated by horizontal vermicular channels, 

 of stelliform arrangement around the narrow, punctiform, central 

 tube channels in vertical position, and their identity with the other 

 well-formed specimens would scarcely be supposed, if all possible 

 gradations from normally formed ones to the disfigured altered 

 specimens were not plentifully found associated with them. Found 

 at Point Detour, Drummond's Island, and in other Niagara out- 

 crops, in association with the former species ; occurs also in the 

 drift of the Lower Peninsula, and in the Niagara group of Indiana 

 and Kentucky. 



Plate XXV. — Fig. 3 represents a calcified specimen found at 

 Louisville, Ky. ; the Michigan specimens are all silicified, with not 

 nearly so well-preserved surface characters. 



