5° 



THE QUARTERLY REVIEW OF BIOLOGY 



of the so-called occipital shield and are 

 broadly overspread by the supraoccipital. 



It would appear that the archaic toothed 

 whales and the zeuglodonts, which had 

 dominated the later Eocene seas, dwindled 

 in numbers almost to the point of extinc- 

 tion by the close of the Oligocene. So 

 far as the record shows, an extraordinary 

 transformation of the odontocete skull 

 took place in the interval of time preceding 

 the beginning of the Miocene, which seem- 

 ingly swept forward with prodigious 

 rapidity, for several families had already 

 acquired their peculiar details of structure 

 by that time. Nevertheless one must not 

 forget that the developmental history 

 of the toothed whales during the Lower 

 and Middle Oligocene is wholly unknown 

 and that there is a surprising scarcity of 

 described forms from the Upper Oligocene. 

 The archaic toothed whales seem to have 

 given rise to the squalodonts, which were 

 the forerunners and perhaps the direct 

 ancestors of the primitive ziphioid and 

 iniid porpoises. These squalodonts con- 

 tinued through the Miocene, attaining 

 their highest development, but perhaps 

 not their greatest size. The sudden ar- 

 rival of delphinoids with highly tele- 

 scoped skulls as well as physeteroids with 

 depressed braincase and characteristic 

 postrostral basin in the Lower Miocene 

 is the most startling evidence in favor of a 

 rapid remodeling of the archaic toothed 

 whale type of skull during the Oligocene, 

 if it can be shown that they are deriva- 

 tives of the later Eocene stock of true 

 whales. One would hardly anticipate 

 so radical a remodeling of a generalized 

 type of skull in the course of one geologi- 

 cal period. 



The assemblage of toothed whales, 

 which begin their career in the Lower 

 Miocene, includes at least five of the 

 recognized families of odontocetes. 

 These families may be listed according to 



their specialization as follows: Squalo- 

 dontidae, Iniidae, Ziphiidae, Delphini- 

 dae, and Physeteridae. Even in the 

 skulls of these early Miocene porpoises 

 the passages leading to the nostrils have 

 been pushed backward to the level of the 

 eyes and the choanae are nearly vertical, 

 an adaptation for breathing in the water. 

 The postorbital constriction has been 

 eliminated on the skulls of all of these 

 extinct porpoises, and the frontal bones 

 are in contact posteriorly with the supra- 

 occipital. The skulls of all known Mi- 

 ocene porpoises are constructed along the 

 same general mechanical lines as those 

 now living, although their rostra may 

 assume quite different proportions. A 

 noticeable tendency toward the lengthen- 

 ing of the rostrum is evidenced in many 

 of the Miocene genera, and this process 

 seems to culminate in the late Miocene or 

 early Pliocene. Porpoises with moder- 

 ately elongated rostra, actually much 

 shorter than the peculiar Miocene por- 

 poises hereinafter discussed, are found in 

 the existing pelagic faunas, but it was 

 not until the later Miocene that the true 

 short-snouted porpoises made their ap- 

 pearance. The history of the several 

 families of toothed whales may be traced 

 briefly and the typical features of their 

 respective groups illustrated by selected 

 genera. 



SQUALODONTIDAE 



The oldest known faunas of pelagic 

 mammals of the Lower Miocene appear 

 to be those obtained from the Patagonian 

 marine formation on the coast of Chubut 

 Territory, Argentine Republic, and the 

 sand pits of Libano and Bolzano, near 

 Belluno, in the Province of Venetia, 

 Italy. Among the peculiar types of 

 extinct whales found in these formations 

 are the shark-toothed porpoises, Pros- 

 qualodon australis (Lydekker, 1894), Pbo- 



