14 PROCEEDINGS OF THE NATIONAL MUSEUM vol.69 



one is the largest. Behind the antorbital notch one or two maxillarj- 

 foramina, smaller than those in the premaxillae, open into the shallow 

 grooves which lead forward. 



On comparing the dorsal surface of the skull of Sotalia tucuxi with 

 that of Kentr'iodon pemix, it was noted that it differed from the 

 latter in that the horizontal cranial plates of the maxillae 

 are relatively wider and that they do not completely sheath the 

 f rontals, for there is a narrow strip exposed between their posterior 

 extremities and the transverse crest of the supraoccipital ; the brain 

 case is relatively larger and the rostrum is not as slender. Keturning 

 again to the skull of Kentriodon pernix it may be noted that the 

 horizontal plate of the maxilla does not completely cover the supra- 

 orbital process of the frontal, and a narrow strip of the outer margin 

 is exposed above the orbit. In front of the orbit the anteroexternal 

 angle of each maxilla is produced, forming an apophysis. The apo- 

 physis of the maxilla is large and broad, projecting beyond the 

 anterior margin of the supraorbital process and overspreading the 

 lachrymal. The lachrymal is closely appressed to the supraorbital 

 process of the frontal and is barely visible from a dorsal view. 



From a dorsal view the frontals are largely concealed by the over- 

 i^preading cranial plates of the maxillae and the slender premaxillae. 

 They are suturally united posteriorly with the supraoccipital, and no 

 trace of an interparietal could be found on either of the skulls. 

 Mesially the frontals meet edge to edge on the vertex and are 

 overspread anteriorly by the nasal bones. The vertex is elevated, 

 relatively small in area, hexagonal in outline, and is formed by the 

 frontals and nasals. On the vertex the posterointernal angle of 

 each frontal is exposed, forming a six sided area of smaller size 

 than the corresponding nasal. Laterally, each frontal sends out a 

 thin platelike extension which underlies the horizontally expanded 

 cranial plate of the maxilla and contributes the roof for the temporal 

 fossa. Farther forward this portion of the frontal is considerably 

 thickened and arched to form a complete osseous roof for the orbit. 



The nasals are rather large, more or less six-sided bones which 

 bound the posterior margin of the entrance to the combined respira- 

 tory passages. As regards shape, the nasals on both skulls resemble 

 each other so closely that they in conjunction with other features 

 may be said to characterize this species. The anterior margin of 

 each nasal bone is deeply notched and the posteroexternal angle 

 is drawn out into a sharp projection. 



Posterior view. — Notwithstanding the distortion produced by the 

 dorso-ventral crushing of the brain case, the original shape of the 

 posterior surface is fairly obvious. The back of the brain case (pi. 

 3, fig. 1) was inflated, but possibly not as much as in Sotalia tucuxi. 



