10 PROCEEDINGS OF THE NATIONAL. MUSEUM. vol.61. 



Just posterior to the scaphoid fossa is a funnel-shaped recess which 

 enters the cranial cavity. It is bounded internally by a conspicuous 

 lateral swelling- of the basiocciptal, which forms a rounded knob; 

 posteriorly by the exoccipital ; and externally by the squamosal. In 

 this cavity the fused tympanic and periotic is lodged. 



A well-marked groove leads downward from this cavity, follows 

 the external side of the basioccipital. and makes a deep groove in 

 the exoccipital, where it terminates on the surface. This is the large 

 posterior lacerated foramen. 





Fig. 1. — Dorsal view of right periotic of Meg.u'tera miocaena X 1, Cat. No. 10300, 

 U. S. N. M. ; LoMPOC, California. 



As it is not the object of the present paper to go into an explana- 

 tion of the homologies of the elements comprising the periotic, a 

 discussion of this is omitted, and the terminology adopted is based 

 upon the work of Lillie ^* in 1910. 



The periotic bone is irregularly V shaped, the apex or opisthotic 

 is not complete, while the anterior, or prootic, resembles a com- 

 pressed three-sided pyramid. On the dorsal face (fig. 1) the 

 prootic is separated from the labyrinthic region by a broad and 



" Lillie, D. G., Observations on the anatomy and general biology of some members of 

 the larger Cetacea. Proc. Zool. Soc. London for 1910, pp. 769-792, pi. 74, text figs. 69-78 

 [discussion of auditory organ of Balaenoptera, pp. 775-781, text figs. 71-72, pi. 74]. 



