124 PALEONTOLOGY OF NEW YORK. 



The character of most emphatic importance in this group is the broad, ele- 

 vated, internal pedicle-disc with thickened lateral margins, but with a thin, 

 tenuous central area, perforated by an oval slit passing directly, not obliquely, 

 through the substance of the area. On the outer surface of the shell this disc 

 is much depressed, interrupting for its entire width the concentric growth-lines, 

 and showing upon its surface only the progressive track of the foramen as it 

 has become closed in its advance from the apex toward the posterior margin 

 with the growth of the shell. 



When we turn to the consideration of the generic value of palaeozoic fossils, 

 passing current under the name Discina, we are met by two serious obstacles : 

 (1) it is rarely that these fossils are preserved so as to show the exact character 

 of the foraminal aperture and area, this condition, no doubt, being largely due 

 to the extreme, often membranous tenuity of the parts immediately surround- 

 ing the aperture, and the ease with which the projecting portions of the area are 

 broken ; (2) the illustrations of these fossils which have been given by authors 

 are, as a rule, drawn on so small a scale, or from such unsatisfactory specimens 

 that it is impossible to form an accurate idea of the character of the aperture 

 and disc. Notable exceptions to this general statement will presently be 

 cited. 



ZiTTEL,* recognizing the three divisions, Discina, Orbiculoidea and Discinisca, 

 as sub-genera of Discina, Lamarck {i. e. (?), the current interpretation of Dis- 

 cina, Lamarck), is authority for the statement that the genus Discinisca ranges 

 from the Siluriaii faunas to the recent. Personally, we have no knowledge of 

 any palseozoic species showing the elevated disc and vertical slit characterizing 

 Discinisca, and are not aware that any author has described and figured a 

 palaeozoic species which can be safely referred to this genus. 



We have before us a very large amount of material representing the palseozoic 

 " Discinas," which has been carefully selected for the purpose of studying the 

 variations in the characters of the pedicle-groove and disc. Among the species 

 well represented are Orbiculoidea conica, Dwight, fi"om the Trenton, D. tenuilamel- 



* Handbuch der Palaontologie, vol. i, p. 667. 1880. 



