LYDEKKEK ON THE GENUS MICROCHOCRUS. 529 



39. Note on the Zoological Position of the Genus MiCROCHCEEUSy 

 Wood, and its Appaeent Identity luith Hyopsodtjs, Leidy. By 

 K. Lydekker, Esq., B.A., F.G.S., &c. (Eead June 24, 1885.) 



In 1846 the late Mr. S. V. Wood described* and figured the palate 

 and left ramus of the mandible of a small mammal from the Upper 

 Eocene of Hordwell, under the name of MicrocJioerus erinaceus, and 

 considered that it was allied to Hyracotherium ; although the specific 

 name appears to indicate that he had a suspicion of other affinities. 

 In 1870 Prof. J. Leidy t proposed the generic name of Hyopsodus for 

 a small mammal from the reputed Upper Eocene of "Wyoming, the re- 

 mains of which were figured and described in a later workt, and 

 were also regarded as indicating an animal allied to .Hyracotherium. 

 A recent comparison of the type specimens of Microchosrus, which are 

 now in the British Museum (No. 25229), with Prof. Leidy \s figures 

 of Hyopsodus has convinced the writer that the two forms are ex- 

 tremely closely related and, in his own opinion, generically identical. 

 The English form, of which the right upper dentition is figured on 

 an enlarged scale in the accompanying woodcut, agrees in size with 

 Hyopsodus vicarius, Cope§, and the resemblance is so close that it 



Microchoerus erinaceus. The Right Upper Dentition, from the 

 palatal aspect, x 2. 



is difficult to point out even specific differences ; the former shows, 

 indeed, a minute accessory column in the valley external to the 

 true main outer columns in the upper true molars, which is wanting 

 in Prof. Leidy's figure of H. paidus, but whether present or absent 

 in H. vicarius cannot be determined from Prof. Cope's figure. In 

 any case the absence or presence of this column would not, in the 

 present writer's opinion, be a character of more than specific value. 

 The figured American specimens of Hyopsodus do not show all 

 the anterior teeth, and in this respect the type of Microchoerus is of 



* * Charlesworth's London Geological Journal,' No.i. p. 5, pi. ii. figs. 1 and 

 3 (1846). 



t Proc. Ac. Nat. Sci. Philad. 1870, p. 110. 



I ' Contributions to Extinct Vertebrate Fauna of the Western Territories ' 

 (Rep. U. S. Geol. Surv.), pp. 75-80, pi. vi. (1873). 



§ Vic'e Amer. Nat. 188o, p. 460, fig. 3. 



2p 2 



