DEVONIC FISHES OF THE NEW YORK FORMATIONS 69 



Little need be added by way of supplementing the above description. 

 The total length of the dental plates rarely exceeds 5 cm, and there is close 

 similarity between tho.se of the upper and lower jaws, excepting that the 

 symphysial margin of the lower is produced into a long and slender descend- 

 ing spine, and the cutting edge of the upper is somewhat less arched or 

 " excavated " than in the lower dental plates. As shown by marks of wear, 

 the tips of the lower dental plates closed outside and slightly behind those 

 of the upper, in the same manner as in Ptyctodus. Dental plates differing 

 but little from those of the present species have been described from the 

 Eifel Devonic, the complete dentition being known in the so called " Ram- 

 phodus " [= R h y n c h o d u s major] of Jaekel' An extensive series of 

 specimens, collected many years ago by Mr Orestes St John from the 

 Cedar Valley limestone of Iowa, is now preserved in the Museum of Com- 

 parative Zoology at Cambridge. 



Formation and locality. Hamilton limestone ; Milwaukee, Wis., and 

 Cedar Valley limestone of Iowa. 



Rhynchodus pertenuis Eastman 



Plate 2, figure 5 



1904 Rhynchodus pertenuis C. J?. Eastman. Am. Nat. 38: 297, text fig. 2 

 Dental plate narrow and elongate, with sharp and extended functional 

 margin and knife blade cross-section ; anterior beak prominent, no sym- 

 physial spiniform process, external surface smooth. 



The unique dental plate upon which this species is founded was 

 obtained from the Chemung of Franklin, in Delaware county, New York, 

 and is now preserved in the State Museum at Albany. The general out- 

 line and proportions of this plate differ from those of all other species, and 

 the absence of a spiniform symphysial process is a very unusual feature. 

 But for the trenchant functional margin and narrow cross-section, the 

 specimen might readily be mistaken for a lower dental plate of Ptyctodus, 

 instead of Rhynchodus. That it is properly a mandibular element, and 



'Jaekel, O. Ueber Ramphodus, etc. Sitzungsber Ges. naturf. Freunde, Berlin, 1903. 

 p. 383-93. Cf. also Am. Nat. 1904. 38: 296 ; Centralblatt fur Mineral. 1900. p. 177. 



