SKELETON. TEETH. 555 



The hard and dense petrous bone is loosely united with 

 the squamosal, and may or may not be ankylosed to the 

 tympanic ; it readily falls out in the dry skull. The 

 tympanic bones are thick and scroll- like. These bones are of 

 great importance to the palaeontologist, for they are found 

 wherever fossil remains of Cetacea occur, and were dredged 

 up from the ocean bed in considerable numbers by the Chal- 

 lenger. The frontal bones are prolonged into a plate on each 

 side which covers the orbit. The squamosal sends forward a 

 strong process which meets this supraorbital part of the frontal. 

 The jugal is usually a slender bone, underlying the orbit and 

 extending from the maxilla to the strong zygomatic process 

 of the squamosal. The maxillae are prolonged forwards almost 

 to the front of the snout, and with them, on their median sides, 

 extend the long premaxillae from the nasal aperture to the 

 end of the snout, where they contribute for a small area to the 

 margin of the mouth. The premaxillae do not bear teeth except 

 in Squalodon and Zeuglodon. The snout is composed of these 

 two bones, and of the vomer and mesethmoid cartilage. The 

 nasals are short and united to the frontal bones immediately 

 behind the nares ; they are often asymmetrical. Distinct 

 lacrymals are present in some whalebone whales, and in the 

 Physeteridae. The nasal passages are almost vertical and the 

 turbinals are vestigial. The pterygoids frequently meet and 

 take part in forming the hard palate. The mandible has a 

 very small coronoid process. The hyoid is a broad plate of bone, 

 and has two pairs of cornua. 



Teeth are sometimes absent. They have conical or com- 

 pressed crowns, are homodont (except Zeuglodon and Squalo- 

 don) and monophyodont, and are often very numerous. 



In the whalebone whales, which have no teeth in the adult, there is 

 in the foetus a set of minute calcified * teeth, some of which are provided 

 with two or even three cusps. Kiikenthal maintains that these are 

 rudiments of the milk dentition, and that he has detected traces of a 

 successional series. Kiikenthal also maintains that he has detected 

 traces of successional teeth in some of the toothed whales, and that the 

 persistent teeth in these belong in reality to the milk dentition. 



The vertebral column is distinguished by the thin disc-like 

 character of the cervical vertebrae which are usually more or 

 less fused together (especially in Balaena) ; by the relatively 

 * Julin, Arch. de. Biologie, 1, 1880. 



