THE PIXNIPEDIA. 359 



Ailuroidea are for tlie most part cligitigrade, but may be plaa- 

 tigrade. In dentition, each of these groups presents forms 

 such as the Bears on the one hand, and the Cats on the other, 

 which may be regarded as extreme modifications, in opposite 

 dii^ections, of the type exhibited by the Dog. 



In the Bears, the dental formula is the same as in the 

 Dogs, but the crowns of the teeth are all more obtuse. The 

 sectorial teeth lose their marked characters, and the molars 

 have flat and tuberculated crowns. The anterior premolars 

 fall out as as'e advances. It is a remarkable circumstance that 

 the teeth of frugivorous and carnivorous Bears exhibit no such 

 differences as would lead to a suspicion of their complete 

 difference of habit, if we were acquainted with these animals 

 only in the condition of fossils. 



The Cats have the dental formula ^. 3-7^ c. ^^ p.m. — ^ 

 m. 5^j = 30. The canines are very long and sharp. The pre- 

 molars are like the Dogs', except that they are sharper, and 

 that the hindermost (the sectorial tooth) has hardly any in- 

 ternal process. The single upper molar is a small tooth with 

 a flat, transversely-elongated crown, and it lies within, as well 

 as behind, the great sectorial premolar. In the lower jaw, the 

 sectorial, or first, molar is the last tooth in the series. The 

 crown is a deeply-bifurcated blade representing the antero- 

 external cusp of the corresponding tooth in the Dog. The 

 " heel " is obsolete. 



While the Bears are among the most comjDletely planti- 

 grade of the Carnivora, the Cats are most entirely digitigrade, 

 and the apparatus for the retraction of the ungual phalanges 

 is so well developed that the claws are completely retracted 

 within sheaths of the integument, when the animal does not 

 desire to use them. To this end the elastic Hgaments are very 

 strong, and the median phalanx is excavated, in order to allow 

 of the lodgment of the retracted phalanx on one side of it. 



b. The Plnni^yedla, or Seals and Walruses, are those Car- 

 nivora which come nearest the Cetacea. The tail is united, 

 by a fold of skin which extends beyond its middle, with the 

 integument covering the hind-legs. These are, in most 

 species, permanently stretched out in a line with the axis of 

 the trunk. The pinna of the ear is small or absent. The toes 

 are completely united by strong webs, and the straight nails 

 are sometimes reduced in number, or even altogether abortive. 

 The inner and the outer digits of the pes are very large. The 

 incisors vary in number and lose their cutting form, llie pre- 



