242 Twenty-third Report on the State Cabinet. 



removal of the shell, and preserving the characters shown in the figures, 

 were collected ; but neither species is likely to prove common. 



The patelloid shell is probably generically identical with some of the 

 Lower Silurian forms which have been referred to the genus Metoptoma 

 PhilL, but it certainly cannot be congeneric with the truncated forms 

 upon which the genus was founded, and which, if we may judge from 

 the figures of the typical forms given by the author, are so similar to 

 the terminal plates of some of our carboniferous " Chitons,^'' as to sug- 

 gest a doubt whether they may not belong to that family. "We there- 

 fore propose for the following species the provisional generic name 

 PALiEACivLEA, in allusiou to their similarity to the recent forms of Acmjea. 



Genus — PAL^ACM^A n. gen. 



Conical univalve shells, having a circular, ovate or elliptical outline, 

 with a more or less elevated subcentral apex, either erect or slightly 

 curving toward one extremity; peristome entii-e, not sinuate. Surface 

 marked by concentric ridges of growth. Internal muscular markings 

 unknown. 



Pal^acm^a typica n. sp. 



Plate 11, figs. 4, 5. 



Shell patelliform, broadly oval in outline, with a depressed conical 

 exterior, and a pointed subcentral apex, situated three-sevenths of the 

 greatest length from one extremity. Surface of the shell marked by 

 several strong folds or undulations parallel to the lines of growth. 



Genus — HYOLITHES Eich. 

 Htolithes gibbosus n. sp. 



Plate 11, flgs. 1-3. 



Shell elongate conical, gradually tapering to an obtuse point, and 

 slightly curving; very depressed, convex on the ventral side, and 

 highly convex and gibbous on the dorsal, the sides being nearly vertical 

 for a short distance from their junction with the ventral margin. Aper- 

 ture rather more than semicircular ; the lip on the ventral portion 

 moderately projecting. 



This species resembles Ui/olithes {T/ieca) jrnmoi'dlalis Hall, from 

 the Potsdam sandstones of Wisconsin, but differs in being much more 

 convex on the dorsal side, and in the less projecting lip of the ventral 

 side of the aperture. The outlines given in figure 3, show the rela- 

 tive convexity of the two species, the outer dorsal line being that of 

 II. gihhosus, and the inner of T. lyr'nnonlialis. 



