72 MOSASAURUS. 



slightly depressed planes, and the mner one is strongly striate at base. The bottom 

 of the exposed pulp cavity of the crown is two lines and a half by one line and 

 three-quarters in diameter. 



34. Two imperfect specimens of teeth, obtained by Dr. Hayden from the Creta- 

 ceous formation Number 4, at the mouth of White River, Nebraska. One of them, 

 represented in Fig. 18, Plate X, is the shed crown of a small tooth, resembling the 

 preceding, except that it is slightly narrower in proportion to its length, and the 

 surfaces, though generally more striate, are almost devoid of subdivisional planes, 

 especially the inner one. Its length is ten lines, the antero-posterior diameter of 

 the oval base four lines and three-quarters, and the transverse diameter four lines. 

 The second specimen. Fig. 19, consists of portions of both fang and crown of a tooth, 

 which appears to have resembled the former one. 



35. The fragment of an upper jaw of the right side received for examination from 

 the Museum of the Smithsonian Institution. It is labelled Leiodon, from near 

 Marion, Alabama, and the adherent matrix indicates that it had been imbedded in 

 a soft cream-colored limestone, resembling the coarser varieties of chalk. It measures 

 four inches along the alveolar border, and contains three fangs, from which the 

 crowns have been broken off. The outer surface is longitudinally straight, vertically 

 convex, and is rough. About an inch above the alveolar border it exhibits a trans- 

 verse row of vasculo-nenral foramina communicating with a dental canal within. 

 The intermediate of the three fangs preserved in the specimen is deeply excavated 

 in the usual manner and contains the crown of a successional tooth. The latter, 

 represented in Fig. 7, Plate XIX, is about ten lines long, and agrees in form Avith 

 those ascribed to Leiodcm. 



36. A supposed pterygoid bone, previously indicated, from near Columbus, Mis- 

 sissippi, discovered by Dr. Wm. Spillman, together with vcrtebrfe, a humerus, antl 

 other remains of a Mosasauroid Reptile, already described. The specimen is repre- 

 sented in Fig. 14, Plate XI ; and I suppose it to be the greater portion of the left 

 pterygoid. It bears some resemblance to a fragment of the lower jaw of a Lepidos- 

 toid Fish, and clearly indicates a species, if not a genus, distinct from the more 

 familiar Mofiasaums of New Jersey. 



The fragment is broken at both ends, though the anterior one appears to be 

 nearly complete ; and in its present state it measures three inches long. The outer 

 border is broken, and the upper transversely convex surface, over an inch in breadth, 

 is also mutilated. The inner border forms a narrow ledge defining the lower from 

 the upper surface. 



The specimen contains five teeth, a vacant alveolus, and portions of two others 

 at the extremities, so that the complement of teeth appears to have accorded >vith 

 that of the pterygoid series of the great Mosasaurus. The crowns are sustained on 

 large osseous pedestals, as in the latter, but instead of being lodged in deep sockets 

 they are rather arranged in a series occupying a broad groove ; the fangs being 

 coossified with each other, with the outer parapet of the bone, and the bottom of the 

 groove, leaving the inner sides for three-fourths their length exposed. 



The anterior four teeth successively increase in size, are then followed by a 

 capacious Aacant socket, and then by another tooth as large as the fourth one. 



