550 THE AMERICAN NATURALIST. {VOC XXXII. 
tively, since it has a present width of 1 cm. as far as it is 
preserved back of the beak. The anterior margin is still about 
1.5 cm. in thickness, but how much more has been abraded 
can only be surmised. This is enough, however, to show that 
the form does not belong to either Ptyctodus or Rhynchodus, 
although it resembles the latter in contour; hence, we have no 
recourse but to admit it as a new species of Palæomylus. The 
transition to Rhynchodus, brought about through thickening 
of the symphysial region and development of a broad triturating 
surface, evidently took place through the species described 
above as R. major and R. rostratus. 
ASSOCIATED ICHTHYODORULITES. 
Rohon,! in his paper on Ptyctodus, mentions the occurrence 
in the Russian Devonian of dorsal fin-spines belonging to the 
so-called “ Chimzeroid type of ichthyodorulites,” as defined by 
Jaekel.2 As no other form with which the remains can be 
theoretically associated is present in the same horizon, Rohon 
suggests that both dentition and defenses may have belonged 
to Ptyctodus. The Russian spines are bilaterally symmetrical, 
triangular in section, slightly curved backward, and are orna 
mented with numerous small tubercles, more or less regularly 
arranged. The posterior face is concave, and bears a double 
series of small denticles. 
The style of ornamentation of these spines is remarkable, and 
we are at once struck with the coincidence that in the Hamilton 
limestone of Milwaukee ichthyodorulites should be found which 
have a similar tuberculated ornament. Several very choice 
examples have been obtained by Messrs. Teller and Monroe, 
one of the most perfect being that reproduced in Fig. 49, the 
property of Mr. Teller. 
This spine has a very graceful curvature, and is of compara- 
tively large size, the length of an arc joining the extremities 
1 Rohon, J. V. Beitrag zur Kenntnis der Gattung Ptyctodus, Verhanal. 
mineral. Geselisch. St. Petersburg, vol. xxxiii, pp. 1—16, 1895. 
O. Ueber fossile lekare, Sitzungsber. Gesellsch. naturforsch. 
Freunde Berlin, No. 7, p. 123, 1890. 
