Cope.] 4d J [May 3. 



selves. In the species successive modifications may be traced from the 

 equine form of the genus represented by H. occidenUile to that which ap- 

 proximates the genus Protohippus, represented by H. seversum. In the 

 former the anterior interior column is larger than tlie posterior, forms a 

 more prominent rib on the inner face of the crown of the tooth, and is 

 flattened or even concave on the inner side. After this form come others 

 where the anterior column is I'ound, and then others where it presents a 

 ridge towards the anterior inner crescent, which in the worn section forms 

 an apex or angle of the outline. The succeeding forms have the two inner 

 columns of equal diameter and prominence, and though the anterior one 

 maintains its distinctness its section projects in an angle towards the adja- 

 cent crescent. Finally we have the form where the suhequal columns are 

 both separated from their adjacent crescents, forming the supposed genus 

 Stylonus Cope. 



The internal columns in all the species become most longitudinally com- 

 pressed in the posterior superior molars. The enamel borders of the lakes 

 are frequently less complex in these teeth. In all the molars the apices of 

 the crescents and columns are narrower in earlier than in later stages of 

 Avear, and the posterior inner column is sometimes separate for a short time. 

 The enamel borders of the lakes become more complex also on wear, till 

 middle age ; tliey then, on further wear, become more simple. 



In the following analytical talde the characters of the fifteen species 

 known to me from North American formations are set forth : 



I. Anterior internal column of superior molars more prominent inwards 

 than the posterior, larger, and not approaching union with the ante- 

 rior internal crescent. Eippotherium Kaup. 

 A. Section of anterior internal column concave or flat on the internal 

 side. Crowns prismatic. 

 a. Lake borders very complex ; the loop nearly or quite isolated. 

 Grinding surface subquadrate ; crown straight ; larger ; crown 25 mm. 



long ; 24 mm. wide H. occidentale. 



Like the last ; smaller ; grinding face 22 by 21 mm H. montezumce. 



Grinding surface oblong, 19 by IG mm. ; crown curved. .H. penmsulatum. 



aa. Lake borders simple ; loop simple, open. 

 Grinding surface oblong, 22 by 19 mm. ; crown nearly straight 



H. nnclairii. 

 AA. Section of anterior internal column oval. 



a. Internal columns wide apart. 



Grinding face subquadrate, 20 by 19 mm. ; lakes of medium complex- 

 ity II. speciosum. 



Like the last, but lake borders highly complex //. plicatile. 



Grinding face oblong, 22 by 21 mm. ; lakes of medium complexity ; col- 

 umn apiculate as in sect. Stylonus ; p. m. iii elongate.. fl! sphenodus. 



Grinding face oblong, smaller, 19 by 17 mm. ; lakes of medium complex- 

 ity ; column entire II. gratum . 



