AQUATIC MAMMALS 



The huge mandibles of mysticetes, being supported to a considerable 

 degree by flotation, do not need a great mass of muscle for simply opening 

 and closing the mouth, but with mandibles of such mass there must be 

 a large supply of reserve muscular power, else a chance encounter with a 

 companion, backed by all the momentum of its huge body, would have 

 disastrous results in the shape of dislocation or fracture. Hence we 

 would expect to find the temporal muscles of considerable size. Of 

 course they are not as powerful relatively as in a terrestrial carnivore 

 which is used to rending sinew and crushing bone, but they are, neverthe- 

 less, well developed. At present in balaenopterids they extend far for- 

 ward of the level of the eye and quite to the base of the rostrum, this 

 being permitted by the position of the eye beneath the intervening supra- 

 orbital plate of the frontal, above which the temporal muscle lies. Kel- 

 logg's illustrations (1928) indicate that there has been progressive for- 

 ward extension of the temporal fossae since the Oligocene. In Patrio- 

 cetus of this period the over-riding of the frontals by the temporal 

 muscles is not apparent or has only just begun; in Miocene Cetothertum 

 the fossae do not quite include the entire supraorbital part of the fron- 

 tals; while in living adult Balaenoptera they apparently extend beyond 

 and onto the maxillae, although in the fetal state they are naturally more 

 restricted. 



There now remains for discussion the question of the telescoping 

 of the cetacean skull. This telescoping may be divided into two cate- 

 gories : the sliding of one bone over another, and the crowding by the two 

 terminal elements of the skull upon intervening elements so that the 

 latter are reduced or eliminated. The latter may well be but a result of 

 the former, for presumably the central elements have been mechanically 

 the weaker and have been largely overcome by a stronger force applied 

 from before or behind. 



Miller (1923) is the latest to have considered at length this process 

 of telescoping. He unqualifiedly subscribes to the belief that it has 

 resulted from the backward push of the water against the head as the 

 animal moved forward, combined with the forward push supplied by 

 the moving body. In other words, that the head has been squeezed 

 between the compressed wall of water against the snout at one end and 

 the neck at the other. To account for conditions in odontocetes he con- 

 siders that these two stresses have been relatively uncomplicated and 

 that the base of the rostrum has overspread the braincase for the reason 

 that originally the sutures in this region were of the squamous type, 

 such as now occur in the fox, and hence would more easily over-ride the 



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