ZOOLOGY. 497 



oridPB were all represented by species. Most of these families will be 

 recoguizetl as still existent, bnt two or three families are only repre- 

 sented by extinct forms, viz : ischyromyida?, mylaganlida}, and i)erhaps 

 the heJiscomys type. 



Use as a factor in the differ eniiat ion of ungulate animals. 



Professor Cope, following ont the line of investigation indicated in 

 this country by Mr. John A.Eyder and Professor Cope himself, seeks to 

 show that the distinction between the odd and even toed types of ungu- 

 lates is due, in great part, to the effect of use of the toes in progression. 

 He recalls that the principal specializations in the ungulates are as fol- 

 lows:* 



1. "The reduction of the number of toes to one in the Perissodactyla 

 (horses, etc.), and two in the Artiodactyla (cloven feet). 



2. "The second hinge-joint in the tarsus of the Artiodactyla. 



3. " The trochlear ridges and keels of the various movable articula- 

 tions of the limbs," whether looking downwards (5 categories) or look-, 

 ing upwards (4 categories). 



It is insisted upon that the trochlear keels that look downwards are 

 much the most prominent and important. The others, enumerated as 

 looking forward, are weak and insignificant, or of a different character 

 from the down-looking ones. " The latter are all projections from the 

 middles of the ends of the respective elements. The up-looking are 

 generally projections of the edges of the bones. Such are the lateral 

 crests of the astragalus and the adjacent edges of the cuboid and nav- 

 icular bones, which cause the emargination of the astragulns in the 

 Artiodactyla. The proximal ridges of the phalanges are very weak, and 

 the concavities in the extremity of the radius cannot be called trochlear, 

 as they are adaptations to the carpal bones." 



Professor Cope then comes to the following conclusions : 



1. " The reduction in the number of toes is supposed to be due to the 

 elongation of those which slightly exceeded the others in length, in con- 

 sequence of the greater number of strains and impacts received by 

 them in rapid progression, and the comi)leraentary loss of material 

 available for the growth of the smaller ones." 



2. "The hinge between the first and second series of tarsal bones in 

 the Artiodactyla may be accounted for by reference to the habits whicli 

 are supposed to have caused the cloven-footed character. Observation 

 on an animal of this order moving in mud shows that there is a great 

 strain anteroposteriorily transverse to the long axis of the foot, which 

 would readily cause a gradual loosening of an articulation." 



3. "The trochlea?. These prominences, which form the tongues of the 

 tongue and groove articulations, exhibit various degrees of develop- 

 ment in the different mammalia, and those of different parts of the 



•Cope (E. D.) On the effects of Impacts and Strains on tlie feet of Mammalia. 

 Am. Nat., v. 15, pp. 542-548. 

 S. Mis. 109 32 



