Cope.] er [Jan. 
by two round foramina. The shaft is abruptly contracted in front of the 
expansion, and is trigonal in se tion. The prefontal extends down- 
wards and forwards, and carries inferior and anterior articular faces ; 
the latter vertically transverse. The postero-inferior portion of the 
ethmoid bears on its posterior extremity a concave articular face, which 
opposes that of the prefrontal. The floor of the brain-case in front is 
supported by a vertical style, which is bifurcate above, and rests on the 
parasphenoid. 
Of the teeth, in general, it may be added that their pulp cavity is rather 
large at the base, but rapidly diminishes in the crown. The mode of 
succession is by direct displacement from below. The young crown rises 
into the pulp cavity, and destroys the vitality of the crown, while the 
root is absorbed. Numerous empty alveoli are to be found in all the 
jaws of this genus, in which examination will often detect the apex. of 
the crown of the young tooth. 
The vertebra in this genus are rather short, but not so much so as in 
sharks. In P. thawmas, nearly eighty dorsals and caudals were pre- 
served ; those without lateral grooves, or cervicals, are not numerous. 
rhaps, not more than four vertebrae supporting the caudal 
ficult to determine, owing to the concealment of the 
There are seven hemapophyses in the 
There are, pe 
fin, though this is dif 
terminal centra by bases of radii. 
support, all flat except the first, which is like those anterior to it. The 
second is articulated freely to its centrum, and is wider than the others. 
Tts condyle is characteristic, being double, and with a foramen between 
it and the produced extremity of posterior margin of the pone. It is 
slightly separated distally from the third, but the remainder are in close 
contact. The radii of the superior lobe of the caudal fin extend at least 
as far down as near the end of the third haemal spine from below. The 
structure of these parts in the P. molossus, are as in P. thawmas, so far as 
preserved. 
As some of the spines are not referable to their precise species in this 
genus, they may be described here. A large compound spine found in 
the blue limestone shale in Fossil Spring Cafion, is composed at the base 
of about twenty-six narrow double rods. A few appear between the 
others beyond the base making thirty-one altogether. They are very 
oblique to the general base, but curve so as to pecome nearly straight, 
and enlarge distally. They terminate ina thickened portion which bears 
an acute edge, which truncates them obliquely, forming the cutting edge 
of the spine. This portion is enamelled ; the edge is slightly convex at 
the base, and slightly concave at a point probably beyond the middle. 
M. 
Length of fragment (12 inches)..---. Pee B ITT Drea bios sighs ets 0.80 
AW iciGlis tit MASGik ws cubis loc cw retvoces wreewces 604 ss Raa leu dg oilre 
Thickness at DAase....1...... eee ta terse ses Hal vais Nias moeas Uae 
.007 
Thickness at broken end an inch from CO FO seo uid vis wie vie 
This is a formidable weapon and could be readily used to split wood in 
its fossilized condition. 
