494 H. W. NORRIS AND SALLY P. HUGHES 



correctly employed. Dr. A. G. Ruthven, of the University of 

 Michigan, collected and fixed by the vom Rath method a speci- 

 men of Dennophis mexicanus in vsouthern Mexico. A specimen 

 of Caecilia gracilis, collected by him in British Guiana, was also 

 contributed. Thanks are due Dr. Thomas Barbour, of the 

 Museum of Comparative Zoology, Harvard University, for his 

 fruitful suggestions and the gift of specimens of Geotrypetes 

 petersii from German West Africa, and Ichthyophis beddomii 

 (glutinosus?) from India. Prof. J. S. Kingsley, of the University 

 of Illinois, kindly loaned his' serial sections of heads of larval 

 Ichthyophis glutinosus. To the trustees of the Bache Fund of 

 the National Academy of Sciences acknowledgment is made of 

 a grant that greatly expedited the completion of this research. 



Two types of cranial structure are found in the caecilians: 1) 

 eye covered or nearly covered by the maxilla, eyeball very rudi- 

 mentary, no optic nerve, no eye-muscle nerves except the abdu- 

 cens, no 'eye-muscles (Caecilia, Herpele) (fig. 10) ; 2) eye not 

 covered by the maxilla, but shows characteristic structure with 

 muscles and nerves (Ichthyophis, Dermophis, Geotrypetes) (figs. 

 11 to 13, 17, 19). In the second group there is a peculiar lateral 

 shelf-like extension of the ethmoid bone, internal septum (pro- 

 cessus conchoides of the Sarasins) , which bears near its free border 

 the tubular passageway of the ventral olfactory nerve (figs. 6, 7, 

 11, 12). In Herpele and Caecilia, and apparently in Hypogeo- 

 phis (Wiedersheim) , the ventral olfactory nerve runs in a canal 

 in the base of the internal septum (fig. 8, Iv.) . The development 

 of a processus conchoides produces other modifications of the 

 nasal topography. Jacobson's organ, which, in Herpele and 

 Caecilia (figs. 8, 9, jo.), connects with the mesial portion of the 

 nasal chamber, in the other type communicates with the lateral 

 portion (figs. 6, 7, jo.). 



