190 



THE QUARTERLY REVIEW OF BIOLOGY 



and Brandt have figured and described a 

 rather large number of cetotheres from 

 Italy and southern Russia. Conditions 

 for the preservation of the mysticetes 

 apparently were more favorable in Bel- 

 gium than elsewhere, for the largest 

 faunas of extinct cetotheres of Upper 

 Miocene and Lower Pliocene age thus far 

 known occur in the Antwerp Basin, and 

 these species are exceptionally well illus- 

 trated in the memoirs of Van Beneden 

 (18 8 5-1 8 8 6). Many of the Upper Mio- 

 cene cetotheres from the Atlantic coastal 

 plain described by Cope are figured by 

 Case (1904). 



RHACHIANECTIDAE 



On comparing the diagrams of the 

 skulls of living mysticetes it will be seen 

 that Rhachianectes (Andrews, 1914) stands 

 apart from other whalebone whales, and 

 is the least modified, and furthermore 

 that the conditions observed are readily 

 derivable from the cetothere skull. There 

 is a well marked interdigitation of rostral 

 and cranial elements, the base of the 

 supraorbital process is abruptly depressed 

 below the dorsal level, the overthrust of 

 the supraoccipital shield has not advanced 

 far enough to eliminate the interparietal, 

 although the parietals are pushed apart, 

 and the frontal bones are broadly exposed 

 in the interorbital region. 



balaenopteridae 



In North America and elsewhere we 

 observe that the modernized types make 

 their appearance toward the close of the 

 Miocene, and that the Upper Miocene 

 witnessed the reduction and extinction 

 of many of the types that preceded them. 

 For a while the cetotheres continued to 

 exist in diminished numbers, but they 

 seem to have disappeared by the close of 

 the Pliocene. The appearance of modern- 

 ized mysticetes in the Upper Miocene was 



not unexpected in view of the previous 

 history of the group. The whalebone 

 whales found in the Upper Miocene 

 diatomaceous earth of California are 

 unmistakably modernized types and show 

 the cumulative effects of specialization. 

 All of the mysticetes obtained from the 

 diatomaceous earth series exhibit affinities 

 more or less remote with species now 

 living in the Pacific Ocean. Extinct 

 balaenopterine whales with skulls as 

 large as any of their living relatives have 

 been excavated in this earth. The little 

 finner, Balaenopera ryani (Hanna and 

 McLellan, 19x4), appears to be related to 

 the living sharp-head finner whale, Balaen- 

 opera davidsoni. The telescoping of the 

 occipital and facial portions of this skull 

 has not advanced as far as in the living 

 species, but otherwise the resemblance is 

 remarkably close. Although Megapera 

 miocaena (Kellogg, i9xx) appears to be 

 related to the existing Pacific Humpback 

 whale, Megapera versabilis, the inter- 

 digitation of the rostral and cranial 

 elements of the skull is not as far advanced, 

 but the forward overthrust of the supra- 

 occipital shield has reached an extreme 

 stage in this process. 



As early in geological time as the 

 Pliocene, the forward movement of the 

 posterior occipital elements had reached 

 an extreme stage in one or more extinct 

 species of the genus Balaenopera. The 

 skull of the Pliocene Balaenopera cortesii 

 (Portis, 1885, pl- 3> %• 35) from Montana, 

 Italy, exhibits a more pronounced forward 

 overthrust of the elongated triangular 

 supraoccipital shield than in any other 

 known fossil or living balaenopterine 

 whale, but it retains the slender outbowed 

 zygomatic processes of the earlier ceto- 

 theres. Other extinct species of balaen- 

 opterine whales not unlike those now 

 living have been found in deposits of 

 Pliocene age in Belgium (Van Beneden, 



