342 GEOLOGICAL SURVEY OF THE TEEEITOEIES. 



are not numerous. There are, perhaps, not more than four vertebrae 

 suijporting the caudal fin ; though this is difficult to determine, owing- 

 to the concealment of the terminal centra by bases of radii. There are 

 seven hcemapophyses in the 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. Its condyle is characteristic, being double, 

 and with a foramen between it and the produced extremity of the pos- 

 terior margin of the bone. 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. 

 molossns are as in the P. thaumus, so far as i>reserved. 



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 Canon, 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 become nearly 

 straight, and enlarge distally. They terminate in a thickened portion, 

 which bears an acute edge, which truncates them obliquely. This por- 

 tion is enameled ; the edge is slightly convex at the base, and slightly 

 concave at a i^oint x)robably beyond the middle. 



Measurements. 



Meters. 



Length of fragment, (12 inches) •. 30 



Width at base 12 



Thickness at , base 012 



Thickness at broken end an inch from edge 007 



This is a formidable weapon, and could be readily used to split wood 

 in its fossilized condition. 



The third species of spine is represented in most of the species, but 

 one series of rays with spine may not be referable to any of them. 

 The latter is flat and curved, the convex edge trenchant beyond the 

 middle. The posterior edge is obtuse but narrow, and exhibits a slight 

 groove on one side medially ; proximally there is a shallow rabbett, 

 whose floor is transversely rugose. Several layers of the tissue of the 

 spine beyond the basal portion are delicately, longitudinally striate. 

 The distal half is broken away. Length of fragment, 1 foot; width, 

 1.5 inches ; thickness at middle, 5 lines. 



The species of this genus may be distinguished as follows: 



a. Teeth without acute edges : i 



Larger maxillaries, 5 ; second premaxillary larger 

 than first; third mandibular large, behind a cross- 

 groove ; last large mandibular followed by 16 — 8 



small teeth P. molossus.- 



Larger maxillaries, 3 ; first j^remaxillary larger than 

 second ; third mandibular small, no cross-groove 

 in front of it ; 20 small teeth behind last large 



mandibular P. thaumas. 



aa. Large teeth with cutting angle in front: 



Teeth large, not compressed P. angulatus. 



