8 ANATOMY OF THE RABBIT 



marmots, cavies, beavers, mice, porcupines. Fossils of both 

 lagomorphs and true rodents have been found in Palaeocene rocks 

 and are reported not to be more similar than are modern repre- 

 sentatives of the two orders. Their greater resemblance to each 

 other than to other orders is recognized by a questionable associ- 

 ation in a group designated "cohort direst In both these orders, 

 the anterior incisors in both upper and lower jaws are modified to 

 form chisel-like cutting organs, 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 loss of intermediate teeth, involving posterior 

 incisors, canines, and anterior premolars. There is also elaboration, 

 often very considerable, of the remaining premolar and molar teeth, 

 of the lower jaw, and, indeed, of the parts of the skull generally. 

 Characteristic of these animals is the extension, both forward and 

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

 jaw has an antero-posteriorly 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 

 less from side to side. Further, the teeth are curved and the an- 

 terior or incisor teeth are provided with open roots, so that their 

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

 cheek teeth of the rat and other rodents living on mixed diets have 

 closed roots, but those of which the food is more difficult to masti- 

 cate, including the Lagomorpha, have open roots as have the 

 incisors. In these respects and in the elaboration of the intestine, 

 especially the caecum, the rodents exhibit the characters of highly 

 specialized herbivores, but in many particulars they are primitive 

 types. For example, they tend to retain the five-toed (penta- 

 dactyl), plantigrade foot, characteristic of primitive mammalia and, 

 indeed, of terrestrial vertebrates, and exhibit also unelaborated 

 cerebral hemispheres in the brain. 



Like all higher or placental mammalia (Infraclass Eutheria), 

 the rabbit is viviparous, that is, the young are born in a more or 

 less advanced stage of development, after being retained through 

 a period of gestation in the maternal uterus, to the wall of which 

 they are attached by a vascular connection, the placenta. In this 

 feature the placental mammalia differ from the marsupial mam- 



