MORRISON MAMMALS: PANTOTHERIA 65 



Amblot hcrium 0-wtn 1871 



1866. Stylodott, Owen, Gfol. Mag., Ill, 199. Non Stylodon Beck 1837. 



187 1. Amblotherium, Owen, Mem. Mes. Mam., 29. 



1871. Achyrodon, Owen, Mem. Mam. Mes., 37. 



1879. Stylacodon, Marsh, Amer. Jour. Set. (3) XVIII, 60. 



1887. Laodon, Marsh. Amer. Jour. Sci. (3) XXXIII, 337. 



1898. Odoutostylus, Trouessart, Cat. Mam., 1247. Non Odontostylus, Gray 1840. 



1899. Trouessartia, Cossman, Rez'. Crit. Pal., May 1899, 30. Non Trouessartia, Canestrini & 

 Kramer, Jan. 1899. 



1899. Trouessartella, Cossman, in Trouessart, Cat. Mam., 1433. 



Definition. — Dental formula I4 Ci P4 M7-9. Premolars similar to those of Lao- 

 lestes, slender, recurved, with no anterior accessory cusp. Metaconid of molars simple, 

 pointed ; paraconid erect, nearly or quite as high as me'' on posterior molars, separated 

 from me'' by a deep V-shaped notch ; pa'', mc* and talonid cusp in a straight antero- 

 posterior line. External cingulum on molars (faint in one English species). Small 

 forms with very slender jaws with pointed, recurved, hook-like coronoid processes. 



Type. — Amblotherium soricinum Owen. 



Distribution. — Purbeck beds, England. Morrison formation, Wyoming. 



Owen's three genera Stylodon, Amblothe- 

 rium, and Achyrodon have elsewhere been shown 

 to be synonymous (Simpson 1928B, p. 129). Stylo- 

 don was proposed first, but it was preoccupied. It 

 was based on the external view of the lower jaw, 

 Amblotherium on the internal view with the proto- Y\g. 26. Amblotherium. Diagrammatic 

 conid revealed but the metaconid broken, Achyro- internal view of right lower jaw. Three 

 don on the internal view with the protoconid hid- ^""^^ natural size. 

 den and the metaconid complete. Eight years after 



Owen's memoir, in which he still recognized all three genera, Marsh discovered an 

 American ally which he named Stylacodon, and when another eight years had passed, 

 in his final summing up of 1887, he named another genus, Laodon. In his great 

 memoir, Osborn considered Stylodon and Stylacodon as synonymous." Not recogniz- 

 ing this synonymy, which was quite correct, Trouessart in 1898 proposed to replace 

 Stylodon by Odontostylus. The latter name was also preoccupied, and Cossman in 

 1899 proposed replacing it by Trouessartia, but this in turn proved to be preoccupied, 

 and was finally replaced by Trouessartella. Three additional names were thus applied 

 to a genus which had already been given five separate names, one preoccupied and four 

 unpreoccupied but synonymous. 



Laodon was characterized by Marsh as follows : 



I . Metaconid greatly reduced in size. 



^' He follows the procedure, not sanctioned by later usage, of using Marsh's name for the genus 

 but ascribing it to Owen, who first described the same form but under a preoccupied name. He calls it 

 "Stylacodon, Owen, 1866" but draws attention to its correct taxonomic history in a footnote. 



