8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



the toothed whales the process (pi. i, fig. la, a. pr.) is widely spread 

 over almost the entire surface of the frontal including the outwardly 

 expanded orbital wing, while in the baleen whales it is narrowly 

 interlocked with the body of the frontal close to the nasal, leaving the 

 orbital wnng uncovered (pi. i, fig. 2a, a. pr.). 



At the same time that it has been telescoped the modern cetacean 

 skull has been subjected to a process of widening, flattening, and 

 basining in the frontal and basirostral regions. There appears to be 

 little if any indication of this flattening or basining among the zeu- 

 glodonts. In most of the modern cetacea it is a noticeable feature 

 of the skull (see particularly pi. 5, fig. 5 ; pi. 6, fig. i ; pi. 7, fig. 3). 

 As the basining centers about the region of the base of the maxillary 

 its details and results may have been modified by the two types of 

 structure that have just been described. In the baleen whales the 

 horizontally expanded supraorbital process or wing of the frontal 

 seems to have been forced down from the normal mammalian position 

 of the process seen in a sea-lion (pi. 2, fig. 2) or a zeuglodont (pi. 5, 

 fig. i) until its anterior half has come to lie against the similarly 

 expanded orbital portion or " horizontal ventral " plate of the maxil- 

 lary (pi. I, fig. 2; pi. 6, fig. 3). The anterior part of the original 

 orbital cavity is thus obliterated (compare pi. i, fig. 2b with pi. i, 

 fig. ih). The two bones are essentially in contact over a wide sur- 

 face, but there is no semblance of fusion or interlocking between 

 them, nor does their very unusual relationship appear to add any- 

 thing to the strength or efificiency of the skull. On the contrary this 

 broad approximating of the two expanded plates is to me more 

 suggestive of a fortuitous and structurally unharmonious adjustment 

 of these particular parts to that general necessity for widening and 

 flattening of the basirostral region which the skulls of all the modern 

 cetacea seem to be subject to. This appearance of mechanical weak- 

 ness arises primarily from the excessive contrast between the very 

 large orbital plate and the relatively minute ascending process by 

 which alone the huge maxillary bone is directly fastened to the frontal 

 (see pi. I, fig. 2a; pi. 3. fig. 3 ; pi. 4, fig. 3) ; it is heightened by the 

 thinness of the plate, the irregularity of its free margin (pi. i, fig. 2h) , 

 and by the frequent presence of vacuities in its substance, features 

 which strongly suggest an advanced stage of degeneration. In the 

 toothed cetacea, however, the entire structure of this part of the 

 maxillary has an aspect of mechanical efficiency and of perfect adjust- 

 ment to free depressing and basining. The enlarged ascending pro- 

 cess, widely expanded both backward and laterally (pi. i, fig. la; 



