630 THE EOCENE FAUNA. 



• 



fJierium. The ancestral relation of Hyracotherium to Anchitherium, which I 

 have proposed,^ seems nevertheless very probable, but requires the intei'- 

 vention of several intermediate forms. In the Lojihiodontidoe, one of these 

 is Pliolophus (Orotherium Cope 1873). In the Chalicotheriidce through which 

 the line probably passes, we have Lamhdotherium, and in the Palceotheriidce, 

 Anchitherium. 



Species. — I am acquainted with six well-defined species of this genus 

 from the Eocene formations of the United States, and there are probably 

 several others. Two have been described from England. A third reputed 

 species ( H. sideroliticum Pict. ) belongs to another genus, as observed by 

 Kowalevsky, and, as I think likely, to the famil}^ Palceotheriidce. Many of 

 the species oi Hyracotherium are represented. by numerous individuals; this 

 is particularly true of the H. angustidens, one of the smallest of them. In 

 giving a comparative table of characters of the species, it is necessary to 

 confine it to the madibular rami and teeth, as it is the part most frequently 

 found, and in some species the only part known. The characters thus ob- 

 tained are as follows: 



Large; last molar, O^.O!! in lengtli; ramus, 0"'.018 in depth at last premolar; a strong 



external basal cingulum ; little anterior ledge ; anterior tubercles well separated. 



H. craspedotiim. 

 Large; last molar, O^.OIS; ramus, 0'".020; anterior ledges not prominent; external 



ciugula H. vasacciense. 



Last molar, 0°.012 ; ramus, O^.OlGo ; second and third premolars, 0'".0135 ; external 



cingula JI. ventorum. 



Last molar, 0™.011 ; first molar, O^.OOST; ramus, 0"'.0]27; second and third premolars, 



O^.OISO H. osbornianum. 



First molar, 0"'.0065 ; ramus, O^.OISO ; second and third premolars, O^.OISS. 



H. angustidens. 

 First molar, O^.OOSG ; ramus, O^.OllS; second and third premolars, O""r0OSO ..H. index. 



In the following table the characters of the superior molars are given 

 so far as they can be ascertained. I have not seen them in H. oshorniannm. 

 and index. 



a. External basal cingulum weak or wanting; internal strong. 



Diastema behind second premolar very short E. craspedotum. 



aa. Both internal aud external basal ciugula well developed. 



' Proceed. Amer. Philo. Soc, May, 1873. 



"'Orohijfpus^' prociioninus Cope, Ann. Eeport U. S. Geol. Siirv. Terrs., Ib72 (1873), p. 647. 



