SQUALODONTID^E 257 



Choneziphius. 1 The rostral portions of crania from the Antwerp 

 and Suffolk Crags, on the evidence of which this genus has been 

 established, agree with those of Mesoplodon in having the premaxillse 

 in contact with the intervening bones throughout the length of 

 their inner surfaces, and also in showing only a very small portion 

 of the vomer on the inferior surface ; they differ, however, in that 

 the mesethmoid cartilage remains unossified, whereby a fistular 

 vacuity remains. In some species the soldering of the inner 

 surfaces of the premaxill* is incomplete. The interorbital region 

 of the skull is flat ; and there are two pits in the nasal region, of 

 which the right is the larger. 



Family SQUALODONTID^. 



Numerous extinct forms, chiefly known by teeth and fragments 

 of crania, may be provisionally placed here, until more of their 

 osteological characters shall be brought to light. They differ from 

 all existing Cetaceans in having the teeth distinctly differentiated 

 into groups, as in the Archseoceti, the posterior molars being two- 

 rooted. The cranium has, however, none of the distinguishing 

 characteristics of the Zeuglodonts, but essentially resembles that of 

 the Odontoceti, especially in the position of the anterior nares and 

 form of the nasal bones. 



Squalodon. 2 All the forms may be included in this genus, the 

 so-called Rhizoprion not being distinct. Dentition : i f , c -J-, simple 

 teeth of the molar series (premolars ?) -, two-rooted molars -f = ^f ; 

 total 60. The double-rooted molars differ from those of Zeuglodon 

 in having the denticulations of the crown confined to the posterior 

 border, or at all events much less developed on the front edge. 

 Very little is known of the structure of these animals beyond the 

 skull and teeth, fragments of which have been found widely 

 distributed throughout the marine Miocene and early Pliocene 

 formations of Europe, especially in the Vienna basin, many parts 

 of France, and the Antwerp and Suffolk Crags. They have also 

 been found in formations of corresponding age in North America 

 and South Australia. A few isolated teeth have been met with in 

 the cave-deposits of Italy, which, if contemporaneous with the beds 

 in which they occur, indicate the survival of the genus into the 

 Pleistocene period. 



Family PLATANISTID^E. 



Under this heading .may be placed three very singular genera, 

 which, though differing considerably from each other, have several 



1 Duvernoy, op. cit. p. 61. 



2 Grateloup, Act, Ac. E. Sci, Bordeaux, 1840, p. 208. 

 17 



