36 LOWER PENINSULA. 



FAVOSITES LIMITARIS, N. Sp. 



Ramified and reticulated stems, from five to fifteen millimeters 

 in thickness, forming horizontally explanate expansions or erect 

 fruticose ramifications. Tubes very thick-walled, opening nearly 

 rectangularly to the surface, with circular orifices, the walls form- 

 ing either a solid, undefined interstitial mass, or, in another state 

 of preservation, the polygonal outlines of each tube are visible 

 on the surface of the interstices as delicate engraved lines. Sev- 

 eral varieties are observed, in regard to the mode of growth 

 and the size of tubes. The tube orifices rarely exceed the diam- 

 eter of one millimeter; often they are smaller, and in some forms 

 they are all equal in a specimen ; others have smaller and larger 

 orifices intermingled. A part of the orifices on the side faces of the 

 stems are often found closed by opercula, situated below the outer 

 edge of the channels ; in the interior parts of the tube channels 

 diaphragms are not regularly developed, and are of rare occurrence. 

 Pores large, distant, and irregularly dispersed. In older stems the 

 tube channels not unfrequently become considerably narrowed by 

 excessive incrassation of the tube walls, while the pore channels gain 

 in length and width, and appear on the surface as vermicular, trans- 

 verse channels connecting the tube channels, which latter are, in 

 their narrowed condition, hardly larger than the connecting pore 

 channels. In certain specimens the orifices are at a slightly oblique 

 angle to the surface, and surrounded at the lower external rim by 

 a raised margin, which approximates this species to the forms of 

 Favositoids comprehended under the name Cladopora, in indication 

 of which similarity of structure I selected for it the name limiiaris, 

 a '^Joining, transitory formy 



The different well-marked varieties, which I have considered as 

 constituting one species, are found in the corniferous limestone of 

 Michigan, Canada, New York, Indiana, and Kentucky, usually in 

 silicified condition ; finely preserved specimens are also found in the 

 drift deposits of Michigan. 



Plate XIII. represents four varieties of the species. Fig. i is the 

 usual form found in the drift ; it is likewise the most common variety 

 in the corniferous strata of Canada West (Port Colborne) and 

 at Caledonia, N. Y. The faintly developed lips at the lower mar- 

 gins of the orifices are perceptible in the figures, and in one speci- 



