16 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 73 



Una the paroccipital process is much better developed than even a 

 postnatal individual of Callorhinus. 



If the paroccipital-mastoid crest of the Zalophus be viewed as an 

 extension ventrad of the bone, there is little muscular stimulus to be 

 seen — nothing save that of the digastric. If it be viewed as chiefly 

 a lateral development, then there is more reason. In Phoca hufida 

 none of the long muscles is attached to the lateral part of the oc- 

 cipital crest, the three muscles of the mastoid process are narrowly 

 tendinous, and the digastric arises from a pit between the mastoid 

 and bulla. This accounts at least partially for the suppression of the 

 paroccipital process as such in the Phoca — for there are no musclps 

 attached thereto — and for the moderate size of the mastoid process, 

 with its three muscles. In Zdloyhus^ however, the powerful cephalo- 

 humeral, the splenius-trachelomastoid insertion, and the sternomas- 

 toid — which always has a significant effect upon its insertion — are at- 

 tached to the latero-ventral part of the occipital crest, and in ad- 

 dition, the digastric arises all along the paroccipital-mastoid crest. 

 There is no attachment at all confined to the mastoid process in its 

 i^stricted sense save the w^eak cleidomastoid. 



The audital bulla in otariids is small, shrunken-looldng and often 

 rugose, extending to form a projecting lip to the auditory meatus 

 directly ventrad of its orifice. There are certain phocids with an 

 intermediate type of bullae, as through Monachus to Sfeyiorhynchus 

 and then Phoca^ the culmination being in Cystophora with its great 

 globose bullae: and in phocids the projecting lip of the meatus is 

 situated more caudad. A fundamental difference in the bullae of 

 the two families is to be seen in the fetal state. In a Gdllorhinus 

 skull (length 96 mm.) the bullae are very small, noninflated, with 

 tympanic ring very distinct, and border of the auditory meatus 

 regular and subcircular. In one of Phoca vitulina (length 113 mm.) 

 the bullae are perhaps 10 times as large, roundly inflated and' with 

 the ecto-entotympanic suture almost obliterated, the two parts com^ 

 bining to form a single evenly rounded surface. A point of great 

 phylogenetic importance is the fact that the border of the meatus is 

 irregular and the bone deeply indented cranioventrad, indicating a 

 phylogenetic difference in the procedure of ossification at this point. 

 This same character, but to a slightly different degree, is seen in very 

 young bears (Ursus) although in these there is no apparent ecto-en- 

 totympanic differentiation, but it even more resembles that found in 

 puppies (Canis). A far different condition obtains in the otter 

 (Lutra) or any mustelid which I have examined. 



In the fetal Callorhinus the mastoid exhibits slight inflation, but 

 this rapidly disappears soon after birth and the bone is apparently 

 solid, and thin save where it projects toward the paroccipital-mastoid 

 crest. A very different condition obtains in Phoca {vitulina), how- 



