42 ON THE EXTINCT MAMMALIA OF 



prolongation backward by a deep notch, the bottom of which reaches as far forward 

 as the middle of the sectorial molar. 



The inferior maxilla. — The lower jaw o^ Hycenodon horridus is intermediate in form 

 to that of the Wolf and Bear. It is relatively as long as in the former, and its back 

 portion is relatively as broad as in the latter. 



The base intermediately is nearly straight or slightly convex, but approaching the 

 angle it is directed slightly downward, and at the symphysis becomes strongly 

 convex. 



The symphysis is longer than in the Bear, convex longitudinally, and somewhat 

 flattened transversely. 



The sides of the jaw are vertical and slightly convex. A row, apparently of three, 

 mental foramina, occupies the side of the chin. 



The back portion of the jaw, in relation with that of ordinary carnivora, is of great 

 breadth in comparison with its height. It is impressed externally with a deep tri- 

 angular masseteric fossa, the longer side of which runs nearly parallel with the base 

 of the jaw as far forward as the back third of the sectorial molar. The fossa is 

 bounded below by a thick rounded ridge forming that portion of the base of the bone. 



The coronoid process is comparatively short, as in the Bear. It has a wider base 

 than in the latter ; is shorter along its anterior sigmoid border, but longer upon its 

 posterior concave border. It approaches more an equilateral triangle than in the 

 Bear, and has its more abrupt apex less hooked posteriorly. 



The condyle has the usual transversely ellipsoidal form of the carnivora generally, 

 and resembles that of the Hysena, etc. 



Dentition. — The number of teeth possessed by Hyainodon horridus is the same as in 

 the Wolf. They also hold the same relative position with one another, excepting 

 that the posterior molars of both jaws continue to diverge to the last. The formula 

 of dentition may be arranged as follows : 



Incisors -, canines -, premolars -. smaller true molars -, sectorial molars -. 



3 1 ^ i' 2 1 



Molar teeth. — The molar series of Hyoenodon is especially remarkable from the 

 absence of the so-called tubercular form of teeth so common in ordinary carnivora. 



The superior premolars, three in number, except the first one, are less separated 

 from each other than in the Wolf. 



The first upper premolar, better developed than in the latter animal, is inserted by 

 a distinct pair of fangs. It has a broad, low, laterally compressed conical crown, with 

 the base most extended behind, and with the posterior border longer than the 

 anterior. 



The second upper premolar, proportionately longer and narrower than that of the 



