1903.] Hay, North Ainerica?i Cretaceous Fishes. 1 5 



wavy, especially near the base, and the rays make an angle of 

 15° with the edge. I regard these differences as important. 



In the American Museum there are several specimens of 

 fins which I refer to P. tenuis. One of these, No. 205 (PI. I, 

 Fig. 2), is 528 mm. long and is accompanied by a portion of 

 the baseosts and a part of the cleithrum. It presents appar- 

 ently 36 rays, including the shortest one at the base in front. 

 As a result probably of maceration and some disturbance be- 

 fore burial, some of the hindermost rays are separated from 

 one another, except immediately at the base. It is probable 

 that others of the hindermost have been wholly removed. 

 At its base, as now found, the fin is 62 mm. wide. The front 

 edge is gently repand in the basal half; but in the distal half 

 the edge has a uniform curve. The edge is everywhere thin 

 and sharp, and is strengthened by a layer of enamel, as in the 

 other species. This layer is disposed more or less in ridges 

 at right angles to the edge; but these do not have the regu- 

 larity and fineness which they present in P. nitida. As in 

 other species of the genus, the anterior edge of the fin is 

 formed by the ends of the rays which successively outcrop at 

 their distal ends. All the rays, except a few of the first, 

 become broader as they proceed outward. The greatest in- 

 crease in width is found in the most posterior rays. At the 

 base they are only about 1.5 mm. in diameter, but distally 

 they may be as much as 5 mm. in diameter. At about the 20th 

 ray we find at its distal end evidences of a division into an 

 anterior and a posterior portion. This separation becomes 

 still more distinct iri the succeeding rays. In another speci- 

 men, No. 215, traces of the cleft condition may be found as 

 far forward as the 12th ray from the front, and is indicated 

 by a narrow furrow, or line of pits. 



In the front of the fin in No. 215 are two holes which are 

 made entirely through the rays, and these, with some fractures, 

 must have been produced before the specimen was covered 

 with the matrix. It is easy to imagine that this fish had 

 been seized and destroyed by some Portheus or some mosasaur. 



The fins of this species resemble those of P. perniciosa in 

 the angle which the rays make with the edge of the fin. As 



