1304 



CLASS MAMMALIA. 



beds ; while Herpetocetus from the same deposits is an allied genus 

 with an egg-shaped tympanic, and a talon to the mandibular condyle, 

 which recalls that of Physeter. The names Amphicetus, Idiocetus, 

 Isocetus, Heterocetus, and Mesocetus, have been applied to Cetaceans 

 from the Belgian Crag, most or all which may apparently be in- 

 cluded in Cetotherium. Lastly, a vertebra from the Upper Eocene 

 of Hampshire has been referred to Balcsnoptera, but this reference 

 requires confirmation. 



Suborder 2. Arcileoceti. — This suborder is confined to the 

 Eocene and Lower Miocene, and may be characterised by the long 

 nasals, and the presence of teeth differentiated into groups in both 

 jaws. 



Family Zeuglodontid^e. — In the one genus of the only known 



family the dental formula is I. -, C. -, Pm. + M. -. The cut- 



3 1 5 



ting-teeth are simple and pointed; but the cheek-teeth (fig. 11 75) 



Fig. 1175. — Zeuglodon cetoides ', from the Middle Eocene of North 

 America. A, Left lateral aspect of cranium, much reduced; b, An upper 

 molar tooth less reduced. (After Gaudry.) 



have two distinct roots, and compressed, pointed 

 crowns with denticulated cutting-edges. The cra- 

 nium is elongated and depressed ; the brain-cavity 

 is small ; the temporal fossae and the sagittal crest 

 are large ; the cranial rostrum is long, and on the 

 sides largely composed of the premaxillae ; the nasals are long, flat, 

 and narrow ; and the external nares are placed more anteriorly than 

 in living Cetaceans. All the cervical vertebrae are free, while those 

 of the lumbar region are unusually elongated ; but the nature of the 

 limbs is not known. In their dentition, as well as in the characters 

 of the skull — especially the long nasals and the forward position of 

 the nares — the Zeuglodonts depart less markedly from the general 

 plan of Mammalian structure than any existing members of the 

 order ; and it is remarkable that the Mystacoceti show a nearer re- 

 semblance in cranial structure to these fossils than is made by the 

 Odontoceti. The one genus Zeuglodon is known from the lower 

 Tertiaries of Egypt, England, and North America ; the remains of 



