PLACENTAL CLASSIFICATION OF MAMMALS. 99 



an indication of affinity is weakened by the fact of a clear 

 transition between the Carnivora, with a deciduate zonary 

 placenta, and the Cetacea, with a non-deciduate diffuse placenta, 

 being afforded by the extinct Zeuglodon. 



But admitting all these difficulties and gaps in our informa- 

 tion, it appears to me that the features of the placenta afford by 

 far the best characters which have yet been proposed for classi- 

 fying the Monodelphous Mammalia, especially if the concomi- 

 tant modifications of the other foetal appendages such as the 

 allantois and yelk-sac, be taken into account. 



Certain orders of the Mammalia, then have a DISCOIDAL 

 DECIDUATE placenta. These are 



1. The PKIMATES which have never more than four incisor 

 teeth above, and as many below. The hallux is always provided 

 with a flat nail (with occasional individual exceptions), and is 

 capable of a considerable amount of abduction and adduction. 

 All the Primates possess clavicles. 



This order contains three sub-orders 



a. The Anthropidse (Man). The dental formula is 

 .2.2 1.1 2.2 3.3 

 *Z2 C LT^2^ m a3 



and the teeth form a nearly even and uninterrupted series. 



The hallux is nearly as long as the second toe, and is 



capable of comparatively slight movements of adduction 



and abduction. The arms are shorter than the legs, and, 



after birth, the latter grow faster than the rest of the body. 



The habitual attitude of the body in standing or walking 



is erect. 



&. The Simiadse. (The Apes and Monkeys.) 



The dental formula as in man, or with pm ^y 



o.o 



The series of the teeth is uneven and interrupted by a 

 diastema. 



The hallux is considerably shorter than the second toe, 

 and is capable of extensive adduction and abduction. The 

 arms may be longer or shorter than the legs, but the latter 

 do not grow faster than the rest of the body after birth. 



c. The Lemuridw. The dental formula varies. The 



H 2 



