80 ROYAL SOCIETY OF CANADA 



Mesohippus assiniboiensis, sp. nov. 

 Plate II, figs. 8, 8a and 8b. 



27id right upper premolar, unworn. 

 Measurements : 



p- a. p. .0185 hy ir. .017, height of tritocone, .0105 height of 

 tetartocone -008. 



This species is larger than M. intermedins, Osborn and Wortnian, 

 and apparently than M. validus, Osborn, from the Protoceras beds of 

 South Dakota. It resembles M. h rachy stylus, Osborn, from the Leptan- 

 ehenia beds of South Dakota, in the great development of the parastyle 

 which is, however, more distinctly separated in the Cypress hills species. 



In the above tooth (p-) the antero-posterior diameter is greatly 

 increased by the separation and large size of the parastyle. The cross 

 crests are short, steep sided and set almost at right angles to the ecto- 

 loph. The two inner cusps (deuterocone and tetartocone) are strongly 

 and about equally developed. The protoconule ^ (anterior intermediate 

 conule) is very much smaller than the metaconide (posterior inter- 

 mediate conule), 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 hypo- 

 style 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 inter- 

 nal ones (deuterocone and tetartocone). 



This species, knowoi 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 M. 

 hrachystylus. It exceeds M. intermedius and ? M. validus in size, of 

 which it apparently more closely resembles the latter. It is distin- 

 guished 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 parastyle, 

 and (4) the intermediate height of the ectoloph. 



' It has been pointed out by Scott (The Evolution of the Premolar Teeth 

 in the Mammals, Proc. Acad. Nat. Sci.. Philadel., Vol. XLIV, 1892) that the 

 anterior and posterior intermediate conules of the premolar tooth are not 

 homolog'ous with the proto — and meitaconules of tfhe molar although they 

 correspond in position. 



