BRACHIOPODA. 



125 



lata, Hall {^Orbiculoidea Forbesi, Nicholson (not Davidson)=Z). clara, Spencer,^ 

 D.so/iterm, Ringueberg), from the Niagara, D.Conradi, Hall, and D. discus, Hall, 

 from the Lower Helderberg, D ampla, Hall, from the Oriskany sandstone, D. 

 minuta. Hall, from the Marcellus shales, D. media. Hall, D. grandis, Vanuxem, D. 

 humilis. Hall, D. Randalli, Hall, D. Doric, Hall, D. marginalis, Whitfield, from the 

 Hamilton group, D. Lodensis, Vanuxem, from the Genesee shales, D. pleurites, 

 Meek, from the Waverly, D. Newberryi, Hall, from the Cuyahoga shales, D. nitida, 

 Phillips, D. Manhattanensis, Meek and Hayden, Iroui the Coal Measures, and 

 man}^ additional species, with several unidentified, probably undescribed forms 

 from vax'ious horizons. 



The pedicle-perforation in these fossils is not, as usually represented, a simple, 

 elongate fissure, extending from I)eneath the apex, one-third, one-half or the 

 entire distance to the posterior margin. On the contrary, just behind the apex, 

 and removed from it by a distance varying with the stage of growth of the 

 animal, is the external opening of a perforation, which passes very obliquely 

 backward through the substance of the shell and opens on the interior surface 

 not far from, but within the margin of the shell, having thus precisely the re- 



FiG. ti^. Vertical section of ihe itodiolc- 

 valve of Discina striata. 

 After Davidson. 



Fig. (U. Vertical sectiou of tiie pcflicle- 

 valve of Orbiculoid&r. 



verse position to that of Discina striata as given by Davidson,* whose figure is 

 here copied, but very much greater obliquity. 



On the external surface of the pedicle-valve, the pedicle-groove, which begins 

 at the apex, intersects more or less abruptly the usual concentric ornamenta- 

 tion of the shell, but it is very narrow, ami its surface generally smooth or 

 with tnint indications of growth-lines. In all instances this furrow begins at 

 the apex; its length, however, in any given species, will, as just noticed, depend 

 on tiie stage of growth, for the pedicle-aperture evidently keeps the same rela- 

 tive distance from the apex in all periods of development. This portion of the 



* Trans. Linne.an Soc, vol. iv. pt. 1, pi. 26, fig. 26. ISSS. 



