HISTORY OF WHALES 



61 



fossa formed by the ankylosed premaxil- 

 laries, which develop an oblique sinistral 

 crest along their line of contact, dividing 

 the fossa into a large right cavity and a 

 smaller left cavity; premaxillaries either 

 closely approximated on the median line 

 anterior to prenarial fossa or completely 

 fused in old individuals; although con- 

 cealed from view the narrow vomerine 

 canal remains open; maxillaries not de- 

 veloping antorbital tuberosities; antorbi- 

 tal notch double; olfactory foramina 

 large; mesethmoid thoroughly ossified; 

 posterior extremities of premaxillaries 

 bent upward behind the narial passages, 

 forming the lateral borders of the high 

 vertex; nasal bones reduced and lodged in 

 deep fossae, which encroach upon pre- 

 maxillary crests; lachrymal fused with 

 frontal, maxillary, and jugal; maxillary 

 and premaxillary with narrow alveolar 

 gutter; vestigial septa between alveoli 

 not visible on many specimens; length of 

 skull in two old individuals about Z7! 

 inches when complete. 



Inasmuch as there are ten recognized 

 living species of the genus Mesoplodon, 

 there can be no objection to the assump- 

 tion that species of these ziphioids were 

 equally if not more numerous in preceding 

 geological periods. Remains of extinct 

 species of Mesoplodon are rather common 

 in the Upper Miocene sands of the 

 Antwerp Basin (Abel, 1905), the Edisto 

 marl of South Carolina (Leidy, 1877), 

 the Red Crag of Suffolk, England (Owen, 

 1870), and the Pliocene deposits of Italy 

 (Vigliarolo, 1894). Unfortunately the 

 majority of the fossil specimens referred 

 to this genus consist solely of rostra, 

 which are subject to age, individual, and 

 sexual variations. Some of these speci- 

 mens differ very little from living species, 

 and one of them, Mesoplodon longirostris 

 (Cuvier, 1836) may have been a precursor 

 of the living beaked whales, Mesoplodon 

 bidens and M. europaeus. 



The palaeontological evidence bearing 

 on the ancestry of the ziphioids indicates 

 that they were derived from ancestors 

 with functional teeth in both jaws. In 

 the course of geological history the maxil- 

 lary teeth have been suppressed except for 

 the not infrequent occurrence of vestigial 

 teeth buried in the gums, the symphysis 

 of the mandible has been shortened, and 

 the mandibular teeth have been reduced to 

 one or two pairs. One of the distinctive 

 features of living ziphioid whales is the 

 presence of distinct jugal and lachrymal 

 bones. The normal mammalian relation- 

 ships of the jugal and lachrymal are 

 transposed, for the former comes to lie 

 in front of the latter on the ventral aspect 

 of the skull. Living ziphioids retain a 

 remnant of the dentary groove, which in 

 some species consists of a longitudinal 

 basirostral groove beginning in a blind 

 pit below the antorbital maxillary tuber- 

 osity and extending forward along the 

 side of the rostrum. As many as 17 to 19 

 vestigial maxillary teeth have been found 

 in the gums of Mesoplodon grayi (Oliver, 

 19ZZ). The antorbital maxillary tuber- 

 osities become by further enlargement the 

 enormous longitudinal crests of Hyperoodon 

 rostratus (Harmer, 192.4). The rostrum 

 has been deepened and solidified by ossi- 

 fication of the mesorostral cartilage. 

 Some of the remarkable alterations that 

 take place in the ossification of the rostrum 

 during growth have been described by 

 Forbes (1893). All of the living ziphioids 

 (True, 1910) exhibit the same peculiar 

 twisting, forward curling, and elevation 

 of the posterior extremities of the pre- 

 maxillaries and the maxillaries. The 

 freely projecting nasal bones come to 

 overhang the narial passages . The median 

 portion of the supraoccipital bone has 

 been raised higher to conform to the 

 unusual elevation of the vertex. Another 

 peculiarity is that the grooved anterior 

 process of the periotic is lodged in a 



