22 GEOLOGICAL SURVFA'' OP TEXAS. 



II (Stylonus) of Ilippotherium* When the first premolar and tliird true 

 molar of the Protohippus perditus are but little worn, the protocone is 

 distinct as in Ilippotherium, but this state of affairs lasts but a short time. 



The temporary' superior molars in this genus and Hippotherium resem- 

 ble somewhat the permanent molars of Anchitherium. That is, the para 

 and metaconules are not united longitudinally on the middle of the crown, 

 but are in line with the jirotocone and hypocone respectively, and are 

 more or less connected with them. Leidy refers to the deciduous molars of 

 what is probabl}' a species of my Sec. II of Ilippotherium, vinder the 

 name of Merychippus insiynist; and to those of a species of Protohippus, 

 under the name of Merychippus mirabilis.X Both belong to species in 

 which the lake borders are more or less plicate, but it is not possible at 

 present to determine accurately the species to which they should be re- 

 ferred. Deciduous teeth of species with more simple enamel borders 

 have been referred to distinct genera under the names of Parahippus, 

 Hypohippus, and Anchippus.§ Those on whicli the first two names were 

 established were suspected by Leidj'^ to belong to the temporary dentition. 

 It is probable that they belong to the Protohippus, while Anchippus may 

 IDertain to a species of Hippotherium. 



In my private collection is a fragment of the left maxillary bone of the 

 young horse from the Loup Fork bed of Colorado, which contains a per- 

 manent molar just protruded, and the crown of a temporary molar about 

 to be shed. The species is probably the Protohippus perditus of Leidy, 

 which was abundant at that locality. The deciduous crown is very sim- 

 ilar in most respects to those figured by Leidy as Merychippus mirabilis. 

 Another fragment of the right maxilla from the corresponding horizon in 

 Wyoming, contains three deciduous molars and the alveolus of the single 

 root of the first. These molars resemble the one described by Leidy as 

 Hypohippjus affinis, but presents a greater degree of wear. In the second 

 (anterior) tooth the paraconule sends forward no process towards the 

 metaconule,thus agreeing with Leidy' s type. The third tooth possesses such 

 a process, thus resembling the type of Leidy' s Anchippus. In the fourth 

 tooth this process is present and is confluent with the metaconule as in the 

 permanent dentition. All of these teeth are considerably worn, and the 

 protocone is confluent with the paraconule in all of them. In Leidy's 

 type of Anchippus the protocone remains distinct, and, to judge from his 

 figure, considerable trituration would be required to bring about a fusion. 

 Hence it may be that the latter tooth belongs to a species of Hippotherium. 



* A review of North American Species of Hippotherium, by E. D. Cope, Pro- 

 ceedings Amer. Philos. Soc, 1889, p. 429. See Plate II, figure 15; first true 

 molar of H. calamarium. 



t Extinct Mammalia Dakota and Nebraska., Plate XVII, figures 3 and 4. 



X Op. cit., Plate XVII, figures 10 and 11. 



§ Op. cit., pp. 311, 312, 313; Plate XVII, figures 7-13. 



