162 MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



In the form of the muzzle aud in the large caudal fulcra this genus resembles Psephurus 

 rather than Polyodon. In its numerous slender fulcra it is, however, like the latter genus, while 

 the anterior fulcra are more scutiform, and extend further forwards than in Psephurus. The prob- 

 lematical opercular bone differs from anything seen in Polyodon, even if it should prove to be a 

 suboperculum, which is present in that genus. It is, however, separate from and below the 

 operculum in that genus, and is separated from the cartilaginous ? quadrate by a cartilaginous 

 peduncle. 



The identification of the true afiflnities of this form is an important step in the history of the 

 Chondrostei. Hitherto our knowledge of the Polyodontidse has been restricted to the two recent 

 genera named, and their relations to the ancient world have been unknown. We can now date 

 the history of the family from the early Eocene period. 



Char, specif. — On comparing this fish with the Polyodon folium, various differences appear be- 

 sides those already referred to. The bones of the skull proper are as large as those of a middle- 

 sized individual of the recent species named, while the muzzle is considerably shorter than would 

 be found ou such individniil. Although the muzzle has been pressed obliquely, the sizes of its axial 

 lenticular elements indicate that but little of its length is wanting. There is not much indication 

 of the free lateral edges characteristic of the Polyodon folium, although the stellate bones are vis- 

 ible near the edge aud at the base. 



To commence at the posterior extremity of the skull : The epiclavicle is more robust than in 

 the Polyodon folium. Its superior extremity is openly notched on the anterior edge, i)robably to 

 fit a corresponding surface of the dermospheuotic. Its external face has a narrow, oblique ridge 

 commencing opposite this notch and extending diagonally across to the posterior edge, with a, 

 gradual inclination. It forms the posterior edge of a groove which widens below and narrows 

 above, reaching to within a centimeter of the articular notch. A similar groove exists in the 

 paddle-fish, but it only extends as high up as the line of the superior border of the operculum. 

 The posttemporal is shorter and more robust than in the paddle fish, and has the same wide, bridge- 

 like connection with the parietal. Its external border is in a straight line with that of the dermo- 

 spheuotic. The parietal bones are lost from this specimen. The dermospheuotic is a more slender 

 bone than in the paddle fish, and the foramen separating it from the frontal bone is shorter and 

 wider. It sends down a postorbital process, which is much like that of the paddle-fish, but is 

 narrower, and ends in three tooth-like processes. There is a small preorbital process, and below 

 it a palmate ossicle, which may have been brokpn from it or may be distinct. Shortly anterior to 

 this, the element terminates in a laciniate suture, with a narrow, straight band, which is coiitiinH)us 

 as a posterior divergent branch of the possible nasal bone. The junction of the latter with the 

 frontal closes the foramen above described in front. The bone exterior to this band, which occupies 

 the place of a prefrontal in the Polyodon gladius, is either wanting or is represented by a spiculuin 

 which has become separated. The muzzle presents several of the lenticular spicnlar bones seen in 

 the living species. Some stellate bones lie about the base of the muzzle, out of place, and a band 

 of them lies on the slab at a short distance on one side. These are characterized by smaller size 

 aud more slender radii than in the paddle-fish. 



The premaxillary bone is gently convex upwards, instead of straight as in the P. folium. The 

 dentaries have a corresponding form. They are widely and deeply grooved on the inner side for 

 Meckel's cartilage, which is covered by a small subtriaugular bone (? splenial) at the symphysis. 

 TLis element is very much smaller than in the paddle-fish. The alveolar edge of the dentary bone 

 is iicute, and bounds a rabbet whose floor is the superior roof of Meckel's cartilage. The hyoman- 

 dibular bone is a robust, flattened rod, and is apparently curved so as to be convex forwards, 

 although this appearance may be due to injury of the specimen. It is in any case shorter than in 

 the paddle-fish, where it is also straight. The operculum is relatively and absolutely larger than 

 in the P. folium. Its anterior inferior border is concave, and is thickened on the descending 

 portion. Its anterior extremity is beveled on the external side for attachment with an element 

 already mentioned, which may be preoperculum. The greater part of the latter is lost. 



Several dentigerous laminae lie among the jaw bones, from which they have been separated. 

 They are concave on the inferior side, so as to embrace the alveolar borders, ijrobably of the premax- 



