8 MEGALOiS'YX. 



analogous circumstances, in the banks of the Oliio, below Evansville, Vanderbnrg County, Indiana, 

 that they are, comparatively speaking, of a very recent date, at least as recent as the origin of most of 

 the existing species of univalve shells now inhabiting the Ohio river and its tributaries. 



The bones of the Megalonyx sent to you for description were not found altogether, or at one time ; 

 but were picked up from time to time, and from year to year, as they happened to be washed out from 

 their matrix, particularly after recent freshets. 



I did not discover any further remains of edentata when I visited the locality last June, but obtained 

 many horns and bones of deer, and probably of other ruminating animals ; but, unfortunately, my collec- 

 tion made at that time has all been lost in the wreck of the Cape May, sunk this summer in the Ohio 

 river, near Mount Vernon. 



Description of the Skull. — The skull of the Megalonyx Jeffersonii in the collection 

 of Dr. Owen has lost its malar bones and pterygoid processes, but otherwise, except- 

 ing a number of fractures and the loss of a few sujoerficial fragments, is in a com- 

 paratively good state of preservation. (Plates I, II, III, VI, Figs. 2, 3.) It is 

 accompanied by the lower jaw, broken in three pieces. (Pis. I, V.) The upper jaw 

 contains all the teeth excepting the second of the left side and the last of the right 

 side ; but the lower jaw has lost all excepting the second and last of the right side. 



In Megalonyx Jeffersonii, as in the Unau, the first tooth of the series, on each side 

 in both jaws occupies a corresponding position, and in comparison with the other 

 teeth gives the idea of its being a true canine. 



Lateral Vieiv of the Shull. — (PL I.) In Mylodon rohustus and in the Ai and Unau 

 'the upper outline of the skull is convex, but in Megalonyx it is nearly horizontal; 

 departing from this course at the forehead where it is depressed, and upon the nose, 

 where it is convex, though comparatively much less than in the Ai. The inion 

 does not slope so much as in the latter and the Mylodon, and the anterior extremity 

 of the face presents a vertically sigmoid outline. 



The temporal fossa ascends to the median line of the cranium, and there termi- 

 nates upon a prominent, rugged, sagittal crest ; and inferiorly it is bounded by a 

 rugged border, which separates it from the surface of origin of the external 

 pterygoid muscle. Posteriorly it rests upon the strong root of the zygomatic process 

 and the pyramidal border of the inion ; and anteriorly its limits are defined by a 

 semi-elliptical ridge, the free extremity of which forms an obtuse post-orbitar 

 process, as in the Ai, and which in the Mylodon and Unau is developed into an 

 angular apophysis. Viewed from its elevated margins, the temporal fossa is con- 

 cave, but independent of these the surface is convex, and it is everywhere rough- 

 ened by superficial decussating ridges, provided for the firm attachment of the 

 fibres of the temporal muscle. 



The zygomatic process projects outward and forward at an angle of about 45°; 

 and it is also directed downward, and terminates with an obliquely truncated and 

 roughened extremity. Its temporal surface forms a concave slope four and a half 

 inches in length from the boundary of the inion. Its outer surface aritero-posteriorly 

 and vertically is concave, and in the latter direction has a greater diameter than in 

 Mylodon ; measuring just in advance of the middle two inches. 



The mastoid process is a conspicuous object in the side view of the head. It 

 forms the postero-inferior angle of the cranium, and is a strong, vertical, conoidal 

 tuberosit3\ It is verticallj^ excavated posteriorly, and curved slightly .anteriorly. 



