46 REPORT ON ZOOLOGY, MDCCCXLIII. 



dentine, non- vascular dentine and cement ; the vascular dentine constituting 

 the axis. Zygoinatic bone with a descending apophysis ; acromion united 

 with the coracoid process." The further subdivision is as follows : 



1st Family, TARDIGKADA. Extremities long, slender, the anterior more 

 or less longer than the posterior; hands di- or tridactylc; feet tridactyle ; 

 toes covered with falcate claws. Zygomatic arch open, tail very short. 

 Genus I. Brcidypus. Genus II. Clwl&pus. 



2d Family, GRAVIGKA.DA. Extremities short, very robust, equal, or nearly 

 so ; hands, penta- or tetradactyle ; feet, tetra- or tridactyle ; one or two outer 

 toes unarmed, adapted for support or walking, the others with claws. Zygo- 

 matic arch closed ; clavicle perfect ; tail of medium size, thick, serving for 

 support. 



Genus III. Megalonyx. Teeth - ? sub-elliptic, the crown hollowed in 

 the centre, with prominent edges ; anterior extremities the longer ; tibia and 

 fibula distinct ; calcaueum long, compressed, high ; claws large, compressed . 

 (1) M. Jejfersonii, Cuv. (M. laqueatus, Harl.) 



Genus IV. Megatherium. Teeth 5-J_i? contiguous, quadrangular, trans- 

 versely sulcated on the crown. Hands tetradactyle ; feet tridactyle ; two 

 outer digits unarmed. Claws large, variously shaped, that on the middle 

 finger very large and compressed. Head of femur entire ; tibia and fibula 

 conjoined at, each end; astragalus hollowed on the outer side superiorly; cal- 

 caueum long and thick. (1) M. Cuvicri (Bradypus giganteus, Paud.) 



Genus V. Mylodon, Orycterotherium, Harl.) Teeth - separate; the first 

 of the upper jaw sub-elliptical, and at a moderate distance from the others, 

 the second elliptic, the third triangular, bifurcate internally ; in the lower 

 jaw, the first elliptical, penultimate quadrangular, ultimate very large, bi- 

 lobed. Extremities equal, anterior with six, posterior with four toes ; two 

 outer toes unarmed, the others with large subconical unequal claws. Head 

 of femur impressed by the round ligament ; tibia and fibula separate ; 

 astragalus flattened superiorly and anteriorly; calcaueum long, thick. 



(1) M. Darwinii, Ow. (2) M. llarlani, Ow. (Megalonyx laqueatus, Harl. 

 Orycterotherium missuriense, Harl.) (3) M. robustits, Ow. 



Genus VI. Scelidotherium, Ow. (Megaloiiyx, Lund). Teeth - , either 

 contiguous or separated by equal interspaces ; upper ones triangular ; of the 

 lower, the first triangular, the second and third sub-compressed, the external 

 surface sulcate, the ultimate the largest, bilobed. Head of femur with a 

 depression; tibia and fibula separate; astragalus with two hollows anterior! v. 

 Calcaneum long, thick ; claws large, subconical. (1) S. kptocepJialum, Ow. 



(2) S. Cuvicri, Ow. (Meg. Cuvicri, Lund). (3) S. Bucklandi, Owen. (Meg. 

 BucUandi, L.) (4) S. minutum, Ow. (Meg. minutus, L.) 



Genus VII. Cfdodon, Lund. Teeth ~-^. Genus VIII. Sphenodon, Lund. 

 Owen is still doubtful whether the name Platyonyx more lately proposed bv 



