PERMIAN VERTEBRATES 7 01 



sometimes confluent at the base. The principal denticles are closely placed, 

 stout, acute, and recurved. 



"Transverse diameter of shaft .0035™; antero-posterior diameter .0025 111 ; 

 the portion of the shaft preserved is straight." 



It is noticeable that the denticles of the outer row become confluent in a 

 low ridge on the lower portion of the spine. 



[No. 6502.] 



Pleuracanthus (Orthacanthus) gracilis Newb. Plate I, Fig. 4. 



Orthacanthus gracilis Newb., Geol. Surv. Ohio, Pal., Vol. II, 

 p. 56. Plate LIX, Fig. 7. 



Orthacanthus gracilis Newb., Cope, 1881, Am. Nat., p. 163. 



"Spine small and straight, about three inches long, very slender and 

 acute ; section circular at base, posterior face and sides flattened above, the 

 angle inclosed by them set with acute, recurved, compressed denticles 

 throughout the upper two thirds of the entire length ; surface smooth or finely 

 striate longitudinally." 



The name Orthacanthus' was used by Newberry only provisionally for 

 spines which were supposed to belong with teeth called Diplodus, and was 

 to be suppressed when the two should be found together. 



It is noticeable that the denticles are fewer and larger than those on the 

 spine of P. quadriseriatus, and that there is but a single row of denticles on 

 each side. 



[No. 6503.] 



Pleuracanthus (Didymodus) compressus Newb. Plate I, Figs. 5^, 

 $b, $c, $d. 



Diplodus (?) compressus Newb., Cope, 1877, P roc - Am. Phil. 

 Soc., p. 54. 



Didymodus (?) compressus Newb., Cope, 1883, Proc. Phil. 

 Acad. Nat. Sc, p. 108. 



Represented in the collection by several imperfect teeth. Cope offered 

 no additional description of the form, contenting himself with the statement 

 that "one with a lateral and median denticles nearly complete, agrees pretty 

 well with the species cited." In 1883 he substituted the name Didymodus, as 

 the name Diplodus was preoccupied, having been used by Rafinesque for a 

 genus of fishes. 



The teeth are much smaller than the species of the same genus found in 

 Texas. 



