No. 6.] AMERICAN MORPHOLOGICAL SOCIETY. 339 



relation, if any, it may have to the age of the individual or 

 to its condition as regards reproduction. The observations are 

 reported in the hope that other observers may have them in 

 mind. If the phenomena are at all general among Mammalia, 

 they should be seen in many laboratories in the usual histo- 

 logical demonstrations. 



Similar phenomena are, of course, well known in several 

 groups of Invertebrata. 



X. THE MAMMALIAN LOWER JAW. 



W. H. RUDDICK AND J. S. KINGSLEV. 



IN no adult mammal, recent or fossil, is the lower jaw known 

 to consist of more than a single bone, and no author, save W. K. 

 Parker, whose observations appear to have been overlooked, has 

 shown the existence of more elements in its development. We 

 are able to confirm Parker in his account and to identify in the 

 mammals the following bones of the non-mammalian groups : 

 (i) articulare, (2) angulare, (3) splenial, (4) dentary. Of these, 

 articulare and angulare unite to form the malleus, while the 

 definitive lower jaw is composed of dentary and splenial. Two 

 cartilages participate in the formation of the lower jaw, --the 

 Meckelian and a second larger cartilage lying external to this, 

 which, like Parker, we homologize with one of the lower labials 

 of the Ichthyopsida. In its ossification this cartilage is strik- 

 ingly similar to amphibian cartilages, and the resulting bone- 

 a part of the dentary - - gives rise to the posterior part of the 

 lower jaw, including the coronoid and articular processes. In 

 the existence of this lower labial is to be found the explanation 

 of the shifting of the articulation of the lower jaw. It is note- 

 worthy that a lower labial occurs in about the same position in 

 the ganoid Polypterus. 



