Megaloimx (Gr. , great, and , claw), an extinct genus of giant edentates, allied to the sloths, established in 1797 by Thomas Jefferson, in a communication to the American philosophical society of Philadelphia, in whose "Transactions" the bones were described by Dr. Caspar Wistar, who first suggested the affinity of the animal to the recent sloths. The first bones were discovered in a limestone cavern in western Virginia, and were referred by Mr. Jefferson, from the large size of the claws, to some carnivorous animal; the original specimens of this, the N. Jeffenonii (Harlan), are in the cabinet of the academy of natural sciences at Philadelphia. These, and other bones found in Tennessee, Kentucky, Mississippi, and Alabama, form the materials of the most complete monograph on the subject, that of Prof. Joseph Leidy, in vol. vii. of the "Smithsonian Contributions to Knowledge" (1855). The skull is about 14 in. long, with the upper outline nearly horizontal, depressed forehead, and convex nose; the sagittal crest prominent and rugged; zygomatic process strong, and temporal fossa rough for the attachment of muscular fibres; the mastoid process strongly marked; the orbital cavity shallow; the hard palate between the three posterior molars 14 lines wide, with a median convexity nearly as prominent as the teeth, becoming almost plane in advance of the third molars, and varying in width from 2 1/3 to 4 in., perforated by large foramina and by a large incisive foramen between the first molars; the occipital foramen circular, 10 lines in diameter, the surface of the foraminal bone being rough for the attachment of powerful muscles; orifice of nose irregularly circular, about 3 in. in diameter; lower jaw about 13 in. long.
The teeth are . long, without fangs, sub-elliptical, of nearly uniform diameter, with the crown hollowed in the middle, and projecting border; as in other edentates, they are deeply excavated from the bottom for the persistent dental pulp; they have no enamel, being composed of very porous dentine in the centre, surrounded by a harder layer of the same which is enclosed by a thinner crust of cementum; the formula is (5/4)-(5/4), the anterior tooth being considerably in advance of the others, in form and position like a canine; they vary in diameter from 8 lines to an inch; the rami of the lower jaw are widely separated, and the symphysis narrow. The bones of the skeleton are strong, though less so than in the allied megatherium; the scapula is about 1 1/2 ft. long, the humerus 20 in.; the thigh bones are relatively shorter and broader than in the sloths and about 21 in. long; the tibico relatively very much shorter than in the sloths, but of greater relative length than in the mylodon; the shaft of the humerus suddenly expands toward the lower extremity, and is pierced by a large foramen; the astragalus like that of recent sloths, 5 1/4 by 3 3/4 in.; the heel bone developed in an extraordinary manner, being long, compressed, and high; the phalanges large and narrow, and armed with powerful claws; the tibia and fibula distinct, and the foot articulated obliquely, the last leading Mr. Lund to the opinion that the animal was a climber; the anterior limbs a little longer than the posterior; the tail strong and solid.
From the study of the toes Cuvier pronounced the animal an edentate; the well marked ridge in the middle of the articulating surface of the last phalanx indicates a more restricted motion than in carnivora, to which Mr. Jefferson referred it; the upper edge extends further back than the lower, preventing the claws from being raised above a horizontal line, but permitting complete flexion below, as in sloths; their form and proportions are also those of edentates; the middle and third fingers are large, with very strong claws, the index being smaller with a less strong claw, and the thumb and little finger rudimentary. This animal was less heavy in form than the megatherium, which it doubtless resembled in its habits; it was probably of the size of a large ox. The bones are found in the pleistocene or drift formations of America, contemporaneous with the elephant and mastodon, and perhaps surviving them; bones of another species are found in Brazil. (See Megatherium).