COMPARATIVE ANATOMY. 



92 



The following is a tabular arrangement of these distinctive 

 points :- 



Artio-dactyla. 



1. Having an even number 

 of toes. 



2. Having no third tro- 

 chanter in the femur. 



3. The' astragalus divided 

 into two almost equal facets. 



4. The pre-molar teeth half 

 as complex as the molars. 



5. The last molar not pro- 

 jecting beyond the palatal 

 bones. 



6. The nasal bones not ex- 

 panding. 



7. The ectopterygoid bone 

 not penetrated lengthways. 



Perissobactyla. 



1. Having an odd number 

 of toes. 



2. Havingathirdtrochanter 

 in the femur. 



3. The astragalus divided 

 into two very unequal facets. 



4. The premolar teeth as 

 complex as the molars. 



5. The last molar projecting 

 beyond the palatal bones. 



6. The nasal bones expand- 

 ing posteriorly. 



7. The ectopterygoid bone 

 penetrated lengthways. 



I will call attention in this Ass's skull to this black cavity 

 in the incisors, which corresponds with what is called the 

 mark in the Horse's tooth. 



In the Horse it disappears in early life. 



In the Ass it continues throughout life. 



It is formed by the alternate layers of dentine and enamel 

 in the tooth structure being turned in as in the finger of a glove. 



In the Horse, not being turned in very deeply, it becomes 

 soon worn away. 



I draw your attention to this, as it is the most important 

 anatomical distinction that exists between the Horse and the 

 Ass, showing how careful the naturalist should be not to over- 

 look any _characteristic however minute. 



I pass over the Mutilata, among which are the whales and 

 dolphins, as I have no specimens to illustrate them. 



The next class the Bruta consists of the armadillos, ant- 

 eaters and sloths, and their dental formula is very irregular. 



The Cheiroptera, or Bats, are of two kinds. The Frugivora 

 or vegetable feeding Bats, and the Insedivora or insect feeding 

 Bats. The teeth of these latter resemble the true insectivora, 

 but vary in number much in the different species. 



The Insedivora consist of the 



Talpidct, Moles 

 ErinaddcE, Hedgehogs. 

 Soriddce, Shrews. 



