34 SYSTEMATIC PALEONTOLOGY 



combination of these two give an enormous extent to the cavity of the 

 mouth. The external nares are located on the top of the head but do 

 not extend vertically downwards as in the Odontoceti^ they extend more 

 backwards and downwards and are roofed by plate-like nasal bones. 

 The ribs are loosely articulated to the bodies of the vertebrae by a 

 single head only. By some these members of this suborder are divided 

 into two families, the Balaenidae and Balaenopteridae, founded 

 mostly on external characters, but Flower and Lydekker consider that 

 intermediate forms have rendered the division an unnecessary one. 

 Bdlaena, the common Eight Whale, and Balaenoptera, the Eorqual, are 

 the best known of the living forms. 



Family BALAENIDAE. 



In 1895, Cope ^ described several new members of the family Balae- 

 nidae. He prefaced his descriptions with a summary of the characters 

 and relations of the Keocene members of the family which it seems 

 valuable to repeat here. He said : " I have remarked that the 

 Mysticete with its single family, the Balaenidae ^ ' would seem to have de- 

 rived their descent from some form allied to the Squalodontidffi, since 

 their nasal bones are more elongated than those of the Odontoceti, and in 

 Plesiocetus ' (Cetotherium) Hhe superior cranial bones show some of the 

 elongation of that family.' This elongation of the superior cranial 

 wall is not seen in the genus Squalodon, but is moderately developed in 

 the genus Prosqualodon of Lydekker, founded on the P. anstralis Lydd. 

 from Patagonia. It is exhibited in a still more marked degree by the 

 genus Agorophius g. n. Cope, which is represented by the Zeuglodon 

 pygmcms of ]\Iiiller, which was referred to Squalodon by Leidy. The 

 form of the skull in this genus approaches distinctly that of Cetotherium 

 of the Balffiuidte, and the permanent loss of the teeth woiild probably 

 render it necessary to refer it to the Mystacocete. 



" Stages of transition from some such genus as Agorophius to the typical 

 whalebone whales are represented by several genera from the Yorktown 

 beds. Theoretically the loss of teeth hy failure to develop would be 

 accompanied by the loss of the interalveolar walls, leaving the dental 



' Proc. Amer. Philos. Soc, vol. xxxiv, 1895, p. 130. 



2 "On the Cetacea," Amer. Nat., voL xxiv, 1890, p. 611. 



