138 MORPHOLOGICAL TYPES 



differs from the articular-quadrate articulation of all other 

 Gnathostomes. Indeed, at first sight, the quadrate and 

 articular appear to be absent from the mammalian skull. On 

 the other hand, whereas other vertebrates have one single bone 

 connecting the tympanic membrane with the fenestra ovalis 

 of the auditory capsule, in the mammal there are three such 

 bones. The one nearest to the fenestra ovalis is perforated 

 and called the stapes (stirrup) ; it is homologous with the 

 columella auris and hyomandibula of lower forms. The next 

 bone is the incus, which is in reality the quadrate as can be 

 shown by its embryological development ; and the last bone 

 is the malleus which is in reality the articular. During the 

 course of the evolution of the mammals, these bones have 

 therefore undergone a remarkable change of function. 



The " hyoid " is a small plate of bone, connected with the 

 periotic by a number of small bones representing the hyoid 

 arch (styloid process). The ceratobranchials of the 1st 

 branchial arch are represented by the posterior horns of the 

 hyoid (thyrohyoid). Elements of the remaining branchial 

 arches are possibly represented in the cartilages of the larynx 

 and of the trachea. 



Teeth. — Equally distinctive of mammals are the teeth, 

 which are of different shape in the various regions of the 

 mouth, a condition termed heterodont as distinct from the 

 homodont condition of lower forms in which all the teeth are 

 alike. A further distinction lies in the fact that the teeth are 

 replaced once only in the mammal (diphyodont condition), and 

 not repeatedly as in lower forms (polyphyodont). There are 

 four kinds of mammalian teeth : incisors, canines, premolars, 

 and molars. The incisors or cutting teeth are situated at the 

 front of the mouth, those of the upper jaw are borne on the 

 premaxillae. The next kind of tooth is the canine or tusk, but 

 it is not present in the rabbit, which is a herbivorous animal. 

 In the dog the canines are well developed ; that in the upper 

 jaw is the most anterior tooth in the maxilla, and the canine of 

 the lower jaw lies in front of that in the upper when the mouth 

 closes. In the rabbit there is a long gap or diastema between 

 the incisors and the premolars. Premolars and molars are 



