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Genus 1. Megalonyx, Jefferson, Cuv. (Syn. Megatherium, Desm., Fischer.) 

 Dentes ^::^^ subelliptici, coronide media excavati, marginibus prominulis. 

 Pedes antici longiores. Tibia at fibula discretse. Calcaneum longum, 

 compressum, altum. Falculse magnse, compressse. 



Species. Meg". Jeffersoni, Cuv. (Syn. Megatherium Jeffersoni, Desm., Fischer. 



Megalonyx laqueatus, Harlan*.) 



Genus 2. Megatherium, Cuv. (Syn. Bradypus, Pander et D'Aiton.) 



Dentes r-rr contigui, tetragoni, coronide transversim sulcata. Manus 

 tetradactyli ; podarii tridactyli, digitis duobus externis muticis. Falculae 

 magnse, diversiformes ; medii digiti maximae, compressae. Femur capite 

 integro ; tibia cum fibula utraque extremitate concreta. Astragalus pagina 

 antica supra excavata. Calcaneum longum, crassum. 



Species. Meg. Cuvieri, Desm. (Syn. Bradypus giganteus, Pander et D'Aiton.) 



Genus 3. Mylodon, Owen. (Megalonyx, Harlanf, Orycterotherium, Harlanj.) 

 Dentes j^j discreti, superiorum anticus subellipticus, e reliquis modice 

 remotus ; secundus elhpticus ; reliqui trigoni pagina interna sulcata : infe- 

 riorum anticus ellipticus ; penultimus tetragonus ; ultimus maximus, bilo- 

 batus. Pedes sequales : manus pentadactyh ; podarii tetradactyli ; utrisque 

 digitis duobus externis muticis, reliquis falculatis : falculse magnae, semi- 

 conicae , inaequales. 



Caput femoris ligamento rotundo impressum : tibia et fibula discretse : 

 astragalus pagina antica supra complanata.: calcaneum longum, crassum. 



Species 1. Myl. Darwinii, O. Maxilla inferior symphyse longiore angustiore ; 

 molaris secundus subellipticus ; ultimus bisulcatus, sulco interno angulari. 



* The species is founded on fossils from Big-bone Cave, Tenessee, described in the Medical and 

 Physical Researches, p. 319-331. The author does not prove the specific distinction of these remains 

 from the Megalonyx Jeffersoni of Cuvier. 



t The lower jaw described by Dr. Harlan, loc. cit. p. 334-335. It is erroneously ascribed to the 

 Megalonyx laqueatus. 



I Proceedings of the American Philosophical Society, vol. ii. No. 20. p. 109. 



