342 Mr. Owen's Letter to the Editors on 



The genus Mylodon is characterized in the "Fossil Mam- 

 malia, of the Zoology of the Voyage of the Beagle," 4to. No. 3, 

 1839, pp. 68 — 72. In the preceding pages, Dr. Harlan's obser- 

 vations on his Megalonyx laqueatus (Aulaxodon or Pleurodon) 

 are quoted in illustration of the genus Megalonyx, and I con- 

 elude by stating, " with reference to the Pleurodon of Dr. Har- 

 lan, after a detailed comparison of the cast of the tooth on which 

 that genus is mainly founded, with the descriptions and figures 

 of the tooth of the Megalonyx Jeffersonii, Desm., in the i Osse- 

 mens Fossiles,' they seem to differ in so slight a degree as to 

 warrant only a specific distinction." — p. 66. 



The fossil described by Dr. Harlan, which belongs to the ge- 

 nus Mylodon, is the subject of the article, p. 334, succeeding 

 that on the Pleurodon or Aulaxodon, in the " Medical and Phys- 

 ical Researches." It is the portion of the lower jaw in the pos- 

 session of Mr. Graves, of New York. This jaw does not, in my 

 opinion, belong to the Megalonyx laqueatus, but to a different 

 genus of Megatherioid animals founded by me on fossils discov- 

 ered by Mr. Darwin in South America. 



If the Megalonyx laqueatus should hereafter prove — as Dr. 

 Harlan thinks the structure of the tooth and knee-joint renders 

 not improbable — to have a claim to generic distinction, the terms 

 Aulaxodon or Pleurodon may not be inappropriate. To my 

 genus Mylodon, they are decidedly inappropriate, since six out 

 of the eighteen teeth, or one third of the entire dental series, are 

 neither grooved nor fluted. 



The sense which I have attached to the term Mylodon, is a 

 mammal with molar teeth only. It may be objected, that canine 

 teeth and incisors are absent likewise in other genera of Mega- 

 therioid animals. But the accepted term Megalonyx is open to 

 the same objection, for the animal so called had not relatively 

 larger claws than the Megatherium, Mylodon, or even the exist- 

 ing Bradypus and Cholcepus. Other Megatherioids besides Sce- 

 lidotherium, are remarkable for the colossal proportions of their 

 hind-legs. There are also quadrupeds as large as the Megatheri- 

 um, and even larger than it. If Dr. Harlan's conjecture as to the 

 generic distinction of his Megalonyx laqueatus prove correct, 

 the same objection might be raised to his proposed names, Aulax- 

 odon or Pleurodon, which would equally apply to many more 

 mammalian genera, both recent and fossil, than would my term 





