482 EXTINCT AND VANISHING MAMMALS 



that to some extent account for our lack of knowledge about 

 them. Cetaceans at sea are difficult to identify if of small size 

 and only briefly seen from a distance. Probably the food of 

 this group is chiefly squids and other cephalopods, and the 

 habit of feeding on these soft-bodied animals has resulted in 

 the degeneration of practically all the teeth, although at the 

 same time with the loss of most of the tooth row, one or some- 

 times two pairs of teeth have been retained and even enlarged, 

 for what purpose one may only conjecture. Thus M. bidens is 

 characterized by a pair of lower teeth, one on each side, in 

 about the middle of the row, large and conical; M. stejnegeri 

 has this tooth much larger; in M . mirus the teeth are smaller 

 and at the tip of the jaw; while in M. layardii the two teeth, 

 although in about the middle of the jaw, are not conical but 

 flattened and straplike, actually curving inward and nearly 

 meeting above the rostrum so that the mouth is capable of 

 very little opening. What the use of such teeth may be is 

 unknown. Since none of the species of the genus is common 

 enough to be of any commercial or economic value, no pro- 

 tective measures are needed nor could they perhaps be made 

 effective if passed. The group needs therefore only passing 

 mention at most, as a possibly declining genus, of which a 

 number of species still remain but of which little is known. 



For convenience, a list of the living species hitherto described 

 is given, exclusive of synonymy. Dr. F. W. True in 1910 

 published an account of the few specimens then available in 

 American museums. Notwithstanding the rather large num- 

 ber of species, the various dental and cranial characters seem 

 on the whole to show their distinctness. 



SOWERBY'S TWO-TOOTHED WHALE 

 MESOPLODON BIDENS (Sowerby) 



Physeter bidens Sowerby, British Miscellany, p. 1, pi. 1, 1804 (near Brodie-house, 

 Elginshire, British Isles). 



Range: North Atlantic. 



BOWDOIN'S BEAKED WHALE 

 MESOPLODON BOWDOINI Andrews 



Mesoplodon bowdaini Andrews, Bull. Amer. Mus. Nat. Hist., vol. 24, p. 203, Feb. 26, 

 1908 (New Brighton Beach, Canterbury Province, New Zealand). 



Range: Unknown except from type region. 



