44 LOWER PENINSULA. 



genus Limaria, is Cladopo7'a vcrticillata of Winchell and Marcy. 

 It is of common occurrence in the Niagara group of Indiana, Wis- 

 consin, and Kentucky, but has not been met with in Michigan. 



LIMARIA LAMINATA, Hall. 



Thin, undose, laminar expansions covered by an epithecal crust 

 on the lower side. Tubes stout-walled, forming a massive inter- 

 stitial surface between the orifices, which is sometimes larger than 

 a tube diameter, other times less. Orifices, if well formed, crescent- 

 shaped and less than half a millimeter in diameter. The convex 

 side of the crescent forms a more or less projecting lip, with two 

 rather obscure, dentiform crests ; on the concave side of the cres- 

 cent another more conspicuous crest occupies the median line. The 

 specimens exhibit on their surface at various intervals certain 

 centres, around which the orifices are disposed in spirally twisted 

 rows ; the concave sides of the orifices are always directed toward 

 these imaginary central points. Of common occurrence in the Ni- 

 agara group of Drummond's Island, and at Point Detour, but many 

 of the specimens have by silicification lost the finer details of 

 structure. 



Plate XVIII. — Fig. 2 lepresents a specimen from Point Detour, 

 of natural size. 



LIMARIA CRASSA, N. Sp. 



Grows in thick, laminar expansions, of undose surface, covered on 

 the lower side by an epitheca. Often several such laminae are su- 

 perimposed, and sometimes two leaves stick together at the epithe- 

 cal side, and a lamina with orifices on both sides is the result. 

 Orifices variable in the same specimens. If normally formed they 

 are kidney-shaped, separated by massive interstitial spaces formed 

 by the thick tube walls. They open obliquely to the surface, with a 

 lip on the outer margin. In other tubes the lip scarcely projects, 

 and the orifices appear as an unsymmetrically oval opening sur- 

 rounded by a massive interstitial surface. Often, also, the tubes are 

 joined, with nearly erect, subangular, less thick-walled orifices. On 

 the inner side of the walls a conspicuous crest projects from the 

 median line, and on the outer walls two others of smaller size fork 



