Green, Chorda Tympani in Selachians. 421 



mandibular ramus receives the typical motor and communis 

 roots and in addition secondary accessions from the acustico- 

 lateralis and general cutaneous systems. Its gill has disappeared 

 completely, but the arch has been enlarged and accordingly 

 the nerve runs far out onto the hyoidean apparatus, some of its 

 branches also extending still farther to the tip of the mandible. 

 This last appHes also to its cenogenetic additions, notably the 

 lateralis component. The anterior hemibranch of the facialis 

 segment is either absent or greatly reduced. In either case its 

 nerve persists in a vestigeal condition. The pharyngeal branch 

 is present and of typical composition, but enlarged, so as to 

 extend far cephalad of its segment into the anterior part of the 

 mouth cavity as the r. palatinus. Between the pre-trematic 

 and palatine nerves another has been interpolated in the facial 

 segment, which runs forward between the hyoid and mandibular 

 arches. It may be extended out upon the mandible in the 

 lower vertebrates, but in the higher it is extended out upon the 

 hyoid and in forms which possess a fleshy tongue it joins the 

 lingual branch of the trigeminus to innervate this organ, thus 

 forming the chorda tympani. The motive for the forward ex- 

 tension of all of these facial branches is the same ; viz. the for- 

 ward growth of the facial skeleton in gnathostomes and the 

 necessity for the innervation of the sense organs about the 

 mouth from the post-oral segments. The evolution of a chorda 

 tympani is only one incident in this progressive specialization 

 of the oral region. 



Our especial thanks are due to the U. S. Fish Commission 

 and the director of its Woods Hole Laboratory, Dr. Bumpus, 

 for the facilities for this research and for many special cour- 

 tesies. We are also indebted to Dr. R. R. Bensley of the Uni- 

 versity of Toronto and to Mr. B. A. Bensley of Columbia Uni- 

 versity for important dissection material. 



