NEW SPECIES OF DIPLODUS LEETLH 491 
prominent and less numerous on the posterior face, and near 
the base they may become obsolete altogether. None of the 
specimens show the full length of the median denticle, but it 
was apparently long and slender. Further material will be 
required before the characters of this species can be adequately 
defined. 
Phoebodus politus Newberry (Pl. VII, Fig. 5) 
The two new species of Diplodus described above differ from 
most Pleuracanth teeth in an important character which they share 
in common with Cladodont dentition, and that is the striation of 
the coronal surface. The concave under surface of the root, 
peculiar projection of the front margin shown in Figs. I, 2, and 5, 
together with the posterior “button,” are features quite as char- 
acteristic of Phoebodus as of the two species of Diplodus just con- 
sidered. As already stated, the form of teeth exhibited by 
Phoebodus is intermediate between Cladodus and Diplodus. Start- 
ing with the latter genus, the transition to Phoebodus is accom- 
plished by imagining the median denticle to increase in size 
until it equals the lateral cones. Further increment in size of 
the median cone, or reduction of the lateral cusps, which 
amounts to the same thing, transforms the tooth into Cladodus. 
It is noteworthy that in Cladodus, when several lateral denticles 
are present, the outermost are almost invariably the strongest, 
and the smaller ones would thus correspond to the intermediate 
denticles of Phoebodus. 
Phoebodus politus and other Devonian species have the inter- 
mediate denticles well developed, as shown in Fig. 5, but in an 
unpublished’ species discovered by Professor W. C. Knight in the 
Permian of Nebraska, and also in the Triassic P. bvodiet, only the 
three principal cones are present. While the coronal surface is 
striated in the earlier species of Phoebodus, it later becomes 
smooth, and the same observation holds true of Dzplodus. All 
three genera under consideration appear to have had teeth 
*The original specimen is listed as ‘‘ Diplodus sp. nov.” in Professor Knight’s 
article on The Nebraska Permian in the last number of this JOURNAL (pp. 372-374). 
