14 THE GENUS ODOB^NUS. 



The otner bones are described as more or less resembling 

 those of the Walrus, and do not much exceed them in size. 

 Some of them are also said to closely resemble corresponding- 

 parts of Alactherium. 



Van Beneden's descriptions and figures of the lower jaw 

 fragment indicate features widely different from those of the 

 corresponding part in the Walrus, especially in the shortness of 

 the symphysis and in the curvatnre of the part represented, but 

 above all in the number, relative size, and form of the alveoli, 

 and particularly in the large size of that of the canine, which 

 must have been almost as highly specialized as in the Sea Lions. 

 That the tusks referred to it by' Van Beneden (those described 

 by Lankester especially, as well as the fragment he himself fig- 

 ures) belong here, there seems to be at least room for reason- 

 able doubt.* The differences presented by the jaw fragment 

 of Tricliechodon as compared with the corresponding part of 

 Alactherium are even still more marked. 



The more obvious characters distinctive of the three genera 

 of the Odohccnidcc, as at present known, may be briefly indicated 

 as follows : 



Synojisis of the GeneraA 



1. Odob^nus. Eami of lower jaw firmly aucliylosed, even iu early life; 



symphysis short. Incisors (in atlnlt) 0; canines 1 1 ; molars 3 3, the 

 last much smaller than the others. 



2. Alactherium. Eamiof lower jaw not anchylosed; symphysis very long. 



Incisors (in adult) 22; canines 1 1; molars 4 4, the last smaller 

 than the preceding ones. 



3. ''Trichechodon" (Van Beneden). Rami of lower jaw (apparently) 



unanchylosed. Incisors 1 1?, very small; canines 1 1, highly spe- 

 cialized; molars 4 4, the first small, the last three much larger and 

 subequal. 



Genus ODOB^NUS, Unne. 



Odolenus, Linne, Syst. Nat., 1. 1735 (ed. F^e), 59 (applied exclusively to the 

 Walrus in a generic sense). Brisson, E^gne Anim., 1756, 48 (used 

 strictly in a generic sense, but embracing "1. La Vache marine 

 OfZo&en)(s"=Wah-us; "2. Le Lamantine J/f." The characters 

 given apply almost exclusively to the Walrus). 



* Van Beneden himself says : "M. Eay Lankaster avait vu en Angleterre 

 diflKrentes grandes dents, provenant du crag et qui difteraient surtout entre 

 elles par leurs dimensions. Nous croyons devoir rapporter ces dents an genre 

 Alactherium.^^ Yet he cites " Trichechodon Mxlcjii Eay Lankaster" as a syno- 

 nym of Trichechodon l-oiinincMi, described by himself much later ! In view 

 of the uncertainties of the case, it is to be regretted that he did not propose 

 a new generic as well as specific name for his Trichechodon honnincJcii. 



t With reference only to the lower jaw, the only known part, in case of 

 the extinct types, readily susceptible of comparison. 



