ON THE STRUCTURE OF THE LABYRINTH ODONTS. 237 



The mandible of Mastodonscmrus has a strong in-n^ardlj projecting process, 

 which supports an extension of the glenoid cavity, and a well-developed post- 

 articular process of Crocodilian form and proportions. 



These differences might serve to arrange the Labyrinthodonts into two or 

 more groups. In Mastodonscmrus, Anthmcosaurus, Trematosaurus, &c. the 

 postarticular process is strong, and projects far backwards. In Archegosaurus 

 the process is short and comparatively weak ; it is wanting in Lo.vomma. 



Mere size will not explain these variations of structure. There is no ex- 

 traordinary difference of size of cranium among the genera mentioned ; and 

 Loxomma, which alone wants the postarticular process, is neither the largest 

 nor the smallest. But the structural difierences are not improbably due to 

 peculiarities of mode of life. The genera which have the ramus of the 

 mandible produced beyond the glenoid cavity have strong conical teeth, very 

 unequal in size, the largest being set at definite intervals. Loxomma, on the 

 contrary, has flattened teeth with two cutting-edges ; and the inequality of 

 size which they present is apparently due to irregular replacement. The first 

 group may have had the habits of many Crocodiles, feeding chiefly on dead 

 bodies or terrestrial animals, and consequently requiring strength in masti- 

 cation rather than special rapidity in opening and closing the jaws. Loxomma, 

 on the contrary, may have been a sort of Gavial among the Labyrinthodonts, 

 a fish-eater, whose supply of food depended upon dexterity in snapping up 

 smaU, quick-moving objects, gaining therefore by a structure of jaw which 

 gives velocity at the expense of force. 



The dentary bone supports a row of teeth — and in Lahyr'mthodon a short 

 inner series also, consisting of one, two, or three large tusks which are confined 

 to the symphysial end. This is also apparently the case with Trematosaurus, 

 and may be trueof other examples, in which the mandible is distorted by 

 lateral compression so as to show tusks apparently in series with smaller 

 teeth. Dendrerpeton acadianum is represented as having in the lower jaw 

 " a uniform series of conical teeth, not perceptibly enlarged toward the front, 

 and an inner series of larger and phcatcd teeth, as in the upper jaw " *. 



A large oval aperture has been observed upon the inner side of the lower 

 jaw, a little posterior to the middle of the ramus. It is bounded by the 

 articular bone above, and by the angular bone below. 8ach an internal 

 mandibular foramen exists in Mastodonsaurus, Trematosaurus, PacJuigonia, 

 Goniocjlyptus, and in undescribed specimens from the Kouper of Warwick! 

 No trace of an external mandibular foramen has been discovered. In Croco- 

 dilia both are present. 



The mandibular symphysis was incomplete, and the rami were united by 

 ligament or fibro-cartilage, if we may judge from their constant separation in 

 a fossil state. In Pteroplax the opposed symphysial ends are expanded by an 

 inwardly directed process from the inferior border of each ramus f. 



A mucous canal has been observed to run along the lower margin of the 

 outer surface of the rami in Pteroplax, Loxomma, and others. A descending 

 canal is strongly marked upon the external surface of the articular and angular 

 bones of some Triassic specimens. The sculpture, commonly present upon the 

 angular bone, may oover the entire subcutaneous surface, as in Loxomma. 



The outer surface of the posterior end of the mandible is overlapped by the 

 quadrate -jugal, and in some cases by the maxilla also. In Ehinosanrus the 

 quadrate -jugal descends for a considerable distance over the mandible, as far 

 as the upper border of the angular bone. 



* Dawson, ' Acadian Geology,' 2nd ed. p. .365. 



t Hancock and Attliey, ' Nat. Hist. Trans. Northumberland and Durham,' vol. iii. p. 70. 



