38 



and about equally developed. The protoconule* (anterior intermediate cusp) is very 

 much smaller than the metaconule (posterior intermediate cusp), and passes posterior to, and 

 beyond the inner end of, an inwardly directed spur from the protocone (antero-external cusp 

 of the premolar, adopting Scott's nomenclature). The ectoloph has a distinct mesostyle, a 

 broadly rounded and well detached parastyle, and strong ribs, of which the anterior one is 

 particularly rotund. The hypostyle tends to separate from the posterior cingulum, to which 

 it remains connected by a stout bar. The cingulum is robust, high and sharp edged behind, 

 low and forming a narrow shelf abutting against the base of the parastyle in front, and is 

 entirely absent within. The external cusps (protocone and tritocone) rise considerably higher 

 than the internal ones (deuterocone and tetartocone). 



This species, known only from the second premolar, in which the size of the parastyle 

 would be expected to be accentuated, appears to approach most nearly in tooth development 

 to the much smaller 31. brachystylus. It exceeds 31. intermtdius and ? 31. validus in size, 

 but apparently, more closely resembles the latter. It is distinguished from the last two 

 species by (1) the greater development of the protocone and deuterocone in this species, 

 (2) the slight development of the protoconule, (3) the more complete separation of the para- 

 style, and (4) the intermediate height of the ectoloph. 



The foregoing species of Mcsohippus are related to, or resemble, previously described 

 species of the genus from Montana and Dakota as follows : — 

 M. westoni, (Cope.) More primitive than 31 latidtns, Douglass, and 31. montanensis, 



Osborn, from the Lower Oligocene, Titanotherium beds. 

 31 prcecocidens, Lambe. Nearly related to and more advanced than 31 loestoni (and 



? 31. montanensis of the Titanotherium beds). 

 31. propinquus, Lambe. Nearly related to and more primitive than 31. bairdi, Leidy, 



of the Middle Oligocene, Oreodon beds. 

 31. brachystylus, Osborn. The type of the species is from the Upper Oligocene, Lep- 



tauchenia beds. 

 31. stenolopltus, Lambe. Approaches closely M. brachystylus of the Leptauchenia beds. 



31. planidens,~L&mbe. Approaches in size 31. intermedins, Osborn and "Wortman, from 



the Upper Oligocene, Protoceras beds. 

 31. assiniboiensis, Lambe. Some resemblance to, but larger than 31. validus, Osborn, of the 



Protoceras beds. 

 It would seem probable then, that the species from the Cypress hills, in their relative 

 degrees of progressiveness, are to be assigned to the horizons of the Oligocene in the following 

 order : — 



ir " ■'; „„ > Lower Oligocene, Titanotherium beds. 



31. pracocidens \ t> > 



31. propinquus Middle Oligocene, Oreodon beds. 



31. brachystylus \ TT ^.,. T , . , , 



31. stenolophus ) Upper Oligocene, Leptauchenia beds. 



ll SntolLsis } U PP er 01i S oce » e > -Protoceras beds. 



* It lias been pointed out by Scott (The Ev< >lution of the Premolar Teeth in the Mammals, Proc. Acad. Nat. Sci. Pliiladel. 

 vol. XLIV. 1S92) that the anterior and posterior intermediate conules of the premolar tooth are not homologous with the proto- 

 and metaconules of the molar although they correspond in position. 



