126 PALjEONTOLOGY OF NEW YORK. 



pedicle-groove may then be regarded as the track left by the advance of the ex- 

 ternal opening of the foramen, closed by later testaceous secretions and quite 

 homologous to the anterior portion of the great depression surrounding the pedicle- 

 slit in DisciNisCA. In certain thick-shelled species, like Orhiculoidea conica and 0. 

 Forhesi (to which reference will again be made under the discussion of the genus 

 ScnizoTRETA, Kutorga), the evidence of the external furrow ends abruptly with 

 the disappearance of the pedicle-groove into the substance of the valve. In 

 general, however, the shell of palaeozoic " Discinas " is very tenuous, and their 

 compression in the process of fossilization often confounds the features of the 

 inner and outer surfaces of the pedicle-area, making them appear continuous. 

 It seems evident that in these fossils the groove upon the external surface is, 

 as a rule, of essentially the same character as that seen in Davidson's figures 

 of Orhiculoidea Forbesi* Dwight's figures of 0. conica,] and those of the latter 

 species given on Plate IV f of this volume ; outside the foramen the concentric 

 lines following without interruption, as on any other portion of the external 

 surface. 



On the internal surface of the pedicle-valve the track of the pedicle-groove 

 extends along nearly the entire radius of the shell. In no species have we 

 found the internal character of this feature so well retained, and showing so 

 clearly the changes passed through from youth to maturity as in an undescribed 

 species,! from the Cuyahoga shales at Berea, Ohio, and in D. Jiitida from the 

 shales of the lower Coal Measures, at Springfield, Illinois. The specimens 

 from these formations frequently preserve^the test without mutilation or much 

 distortion. Early in the life of these species the foramen has penetrated the 

 internal surface near the apex, and whatever groove has been made upon the 

 outer surface by the radial progress of the foraminal aperture is also marked on 

 the inner side, and is usually somewhat calloused, conspicuously so in adult 



* Silurian Bi-acliiopodii, pi. xii, figs. 14, 14 b, 15, IS. 



t American Joui'nal of Science, 1880, pi. xxi, figs. 1-5. 



t This sjiecies has come to us from various quarters labelled " DisciTia Newherri/i, Hall." With the 

 latter however it does not agree, the pedicle-area not being elevated nor so broad as in that species, and 

 the bi'achial valve rather more convex with its apex nearer the center. The original specimens of D. Wew- 

 berryi are from a ferruginous sandstone, 110 feet below the congloraei-ate at Cuyahoga Falls ; those of the 

 species in (juestion, which may be termed Orhiculoidea Herzeri, from greenish-black shales at Berea, Ohio. 



