OCT. 1906. FOSSIL FISH-SPINE, PHLYCTAEN ACANTHUS TELLEBI. 
163 
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 backwards, and are ornamented 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 tuburculated ornament. Several very choice ex- 
amples have been obtained by Messrs. Teller and Monroe, one of 
the most perfect being that reproduced in Fig. 49 (Plate 1, this 
paper), the property of Mr. Teller. 
This spine has a very graceful curvature, and is of compara- 
tively large size, the length of the arc joining the extremities on 
the anterior margin being 20 cm. The width where it is broken 
off below, which is not far distant from the beginning of the 
exerted portion, is 5.5 cm., and the maximum thickness at this 
point is 5.5 mm. The spine is extremely compressed laterally, 
both sides being almost flat. There is no strongly marked anterior 
keel. The posterior face is slightly sulcated, and each side of the 
sulcus is set with closely approximated tubercles of somewhat 
larger size than those occurring elsewhere. The bottom of the 
sulcus is traversed by a faint longitudinal ridge, triangular in 
section. 
The individual stamp imparted to this spine by its flattened, 
arcuate shape is heightened by its peculiar ornamentation. The 
lateral faces are beset with numerous small tubercles not having 
a very definite arrangement, but in some specimens showing a 
tendency to become parallel to the anterior and posterior margins. 
One of Mr. Monroe's spines has the tubercles disposed more 
numerously along a series of parallel grooves, situated some dis- 
tance apart, the whole presenting a more or less concentric 
appearance, and indicating successive growth stages in the devel- 
opment of the organ. The appearances indicate that the inserted 
portion tapered gradually towards the base, but this region itself 
has not been recognized in any of the specimens thus far ex- 
amined. Most of the tubercles have been worn down smooth to 
their bases, or are evenly rounded on top, but a few retain traces 
