Zoological Position. 7 



The various species constituting this family are distinguished by- 

 several features, including the imperfect development of the clavicle, longer 

 ears and limbs, and the presence of a distinct although greatly reduced 

 tail, from the Picas or Tailless Hares (OcIiodontidcB) of the mountain- 

 ous districts of Central Asia and of North America (Rocky Mountains). 

 The two families are allied, however, in the possession of a common 

 feature, namely, the presence in the upper jaw of a second pair of incisor 

 teeth. This feature distinguishes the sub-order Duplicidentata from that 

 of the Simplicidentata, the latter containing the majority of rodents 

 and embracing all forms with a single pair of upper incisors. 



The mammalian order Rodentia, to which the family belongs, con- 

 tains a very large assemblage of forms — the Squirrels, Marmots, Cavies, 

 Beavers, Mice, and Porcupines being among the more familiar. This 

 order is distinguished by the modification of the anterior incisors in both 

 upper and lower jaws to form chisel-like cutting organs, the teeth having 

 their enamel layer disposed chiefly if not wholly on their front surfaces, so 

 that they remain in a permanently sharp condition. This modification 

 is associated with an extensive obliteration of intermediate teeth, com- 

 prising posterior incisors, canines, and anterior premolars; also with 

 elaboration and often great complexity of the remaining premolar and 

 molar teeth, the lower jaw, and, indeed, the parts of the skull 

 generally. Characteristic of these animals is the extension, both for- 

 ward and backward, of the jaw-musculature. The articulation of the lower 

 jaw exhibits an elongated articular process fitting into a corresponding 

 longitudinal fossa on the skull, the jaw being able to move forward and 

 backward in addition to vertically and from side to side. The teeth are 

 further arcuate in shape, and are provided with open roots, so that their 

 growth is not limited, as it is in the majority of mammals. The rodents 

 are in many particulars primitive types. For example, they tend to 

 retain the five- toed (pentadactyl), plantigrade foot, characteristic of 

 primitive mammalia and, indeed, of terrestrial vertebrates, and exhibit 

 also unelaborated cerebral hemispheres in the brain. In other respects, 

 however, as in the rodent characters above-mentioned and in the 

 elaboration of the intestine, especially the caecum, they exhibit the 

 characters of highly specialized herbivores. 



Like all higher or placental mammalia (Placentalia), the rabbit is 

 viviparous, the young being retained through a period of gestation in the 

 maternal uterus, to the wall of which they are attached by a vascular con- 

 nection, the placenta. In this feature the placental mammalia differ 

 from the marsupial mammalia (Marsupialia) of Australia and South 

 America, the latter being viviparous, but, with one exception, without 

 placenta; also from the egg-laying mammalia (Monotremata) of Aus- 

 tralia, the latter being oviparous, like the majority of the lower, reptilian 

 forms. These three sub-classes of mammals are united, however, by the 

 common features of the class Mammalia. They are warm-blooded 

 animals, provided with a complete double circulation, and with a hairy 

 investment for the surface of the body. In all, the young are nourished 

 for a time after birth through the secretion of modified cutaneous, milk- 

 producing, or mammary glands. 



