170 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
is represented a little too conspicuously. In Figure 39 of the same plate is 
shown a tooth belonging to another species of Phoebodus. <A good figure of 
P. politus Newb., from the Cleveland Shale of Ohio, may be found in the 
Journal of Geology, Vol. VWII., 1899, p. 492. 
Formation and Locality. — Permo-Carboniferous; Blue Springs, Nebraska. 
PETALODONTIDAE. 
Fortunate discoveries of Janassa afford the means for a clear understanding 
of the dentition and form of body in the ray-like creatures belonging to this 
€ 
G 
Fia. 3. 
Diagram showing arrange- 
ment of upper and lower 
dentition in Janassa bitu- 
minosa Schloth. (Slightly 
modified after the resto- 
rations by Hancock and 
Howse, and O. Jaekel.) 
x 4. 
family. Janassa exhibits a ray-shaped trunk cov- 
ered with smooth, rounded, quadrate granules, and 
large pectoral fins which extend forward to the 
head, the pelvic pair being separated from them by 
an interspace. There are no fin-spines, the mouth- 
cleft is very narrow, as in rays, and the tail is 
slender. There can be no doubt that forms like 
this, or like Tamiobatis, Copodus, Psammodus, 
Archaeobatis, etc., were early approximations to 
the modern ray type, whether we consider them 
as genetically related to the latter or not. 
The dentition of Janassa, as determined with 
entire accuracy by Hancock and Howse in J. bitu- 
minosa, is similar in both jaws, and consists of a 
median or symphysial, and three pairs of lateral 
series, each having from four to seven teeth, the 
lateral series diminishing regularly in size from the 
center outwards. The lower dentition is more 
strongly arched and at the same time less extended 
from side to side than the upper, and the cutting- 
margins of the lower functional teeth bite inside 
those of the opposite jaw. The teeth of the outer- 
most lateral series in the upper jaw slightly exceed 
those of the corresponding lower rows in width. 
The manner of succession is peculiar in that the 
oldest-formed teeth, after they have ceased to be 
functional, become piled upon one another in front 
of and away from the oral margin, thus affording 
firm support for the functional ones (cf. Text-fig. 3). 
The teeth of each series are closely wedged to- 
gether and interlock with those of adjoining rows, 
the whole forming a very compact mass. 
The arrangement of teeth in Janassa is well illustrated in the figures given 
