38 
CHIMiEROIDEI. 
Oral surface forming a narrow oblique knife- 
edge, with no differentiated tritors, but 
baying a lamellar-punctate structure 
witbin tbe outer wail . 
Rhynchodus, Newberry. 
II. Sympbysial surface relatively very broad; 
tritors punctate. 
Oral surface triturating, with a single inde¬ 
finite tritoral area. 
Rcilceomylus, gen. nov. 
Genus PT1TOTOBUS, Pander. 
soy a : S>o 'J- [Ctenodipt. devon. Syst. 1858, p. 48.] 
Syn. Aulacosteus , E. yon Eicbwald, Geognosy of Russia (in Russian), 
1846 (according to Letb. Rossica, vol. i. 1860, p. 1548). 
Rinodus, Newberry & Wortben, Pal. Illinois, vol. ii. 1866, p. 106. 
In tbis genus tbe tritoral areas are so much harder than the rest 
of the tooth that they are often preserved in a rolled state after the 
removal of the surrounding tissue. Such is the condition of all 
specimens hitherto described, except the originals of Pander’s pi. viii. 
figs. 10, 12, which exhibit the symphysial region. Specimens in 
the School of Mines, St. Petersburg, the University of St. Petersburg, 
and in the Museum of Comparative Zoology, Cambridge (Mass.), 
prove that the teeth always assumed the form noted above in the 
}h of a tooth in the 
A 
Tooth of Ptyctodus obliquus, Pander, nat. size; Devonian, Russia.—A, inner 
aspect, showing symphysis, the base enveloped in matrix. B, oral aspect, 
the tritors marked by transverse lines. 
first-mentioned museum, showing tbe inner and oral aspects, is 
given in tbe accompanying wmodcut (fig. 5). 
