AQUATIC MAMMALS 



brain capacity of a porpoise is relatively much larger than of a finback 

 whale. In the former at least the brain shows evidence of having been 

 compressed in an antero-posterior direction and is broader than long, 

 this having been caused evidently by the recession of the anterior cranial 

 elements against the posterior ones; and one result of this great brain 

 size is the curvature of the occipital plane. In effect, then, the stimulus 

 for a large brain in the porpoise has been greater than any muscular 

 stimulus which might have been productive of a flat, ridged or crested 

 occipital for the accommodation of unusually powerful muscles, and 

 greater than this endocranial stimulus in large baleen whales. In the 

 sperm whale, however, the osteological crowding that has resulted from 

 the frontal basining for the accommodation of the spermaceti organ has 

 forced the cerebral cavity to occupy a restricted space at the apex of the 

 angle formed by the vertical part of the occipital elevation and the hori- 

 zontal rostral portion. I judge that the size of the brain in an adult 

 Physeter is not at the most over four or five times as great as in the 

 porpoise Tnrsiops, and the foramen magnum passes downward (in an 

 anterior direction) at an angle of more than 45 degrees. 



Weber (1904) has stated that the cetacean supraoccipital is paired, just 

 as it is in Tatusia (Edentata) and Erinacetis (Insectivora) . 



The occipital conditions in the Mysticeti are very different indeed from 

 those in the Odontoceti. Tilting of this element in the former group is 

 excessive and the forward inclination of the occipital plane more accen- 

 tuated than in any other mammal. In whales of the balaenopterid type 

 this tendency has forced the parietals forward so that considerable por- 

 tions of them surmount the frontals, and the occipital covers all but the 

 most anterior part of the parietals. As is frequently remarked, in whales 

 of this type (including Sibbaldus) parts of the nasals, premaxillae, maxil- 

 lae, parietals and frontals occur in one transverse plane. Among living 

 mysticetes the large balaenid whales (Balaem and Ei/balaeiia) usually 

 are said to exhibit the most pronounced tilting of the occipital, but this 

 statement needs qualification, as explained elsewhere. 



It is difficult to decide whether the occipital overthrust in mysticetes is 

 now actually on the increase or wane. Kellogg (1928) has remarked 

 that a greater overthrust occurred in one of the Pliocene balaenopterids 

 than in any whale since. He is also of the opinion that in mysticetes 

 the forward overthrust of the occipital elements has not had precedence 

 over the backward interdigitation of median rostral elements, for in 

 Miocene cetotheres there was less occipital tilting and more interdigita- 

 tion of rostral with cranial elements. But these details hardly should 

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