Geological Society. 309 



the Amphibia natantes of Linnaeus, in the soft mucous membrane 

 which covered the alveolar margin, and subsequently became fixed 

 to the bone by anchylosis, as in the Pike and Lophius. No remains 

 of the locomotive organs of the L. leptognathus have yet been found. 

 Labyrinthodon pachygnathus. — In detailing the remains of this spe- 

 cies, consisting of portions of the lower and upper jaws, an anterior 

 frontal bone, a fractured humerus, an ilium with a great part of the 

 acetabulum, the head of a femur, and two unguial phalanges, Mr. 

 Owen dwells on further Batrachian characters and certain peculi- 

 arities of structure, and shows the points in which it agrees with the 

 L. leptognathus. A portion, nine and a half inches long, of a right 

 ramus of a lower jaw is first described ; and in addition to the cha- 

 racters common to it and the fragment of the lower jaw of the 

 L. leptognathus, in the structure of the angular and dentary pieces, 

 the author shows that the outer wall of the alveolar process is not 

 higher than the inner, as in Frogs and Toads, the Salamanders and 

 Menopome, in all of which the base of the teeth is anchylosed to the 

 inner side of an external alveolar plate. The smaller serial teeth 

 are about forty in number, and gradually diminish in size as they 

 approach both ends, but chiefly so towards the anterior part of the 

 jaw. The sockets are close together, and the alternate ones are 

 empty. The great laniary teeth were apparently three in each sym- 

 physis, and the length of the largest is considered to have been one 

 and a half inch. A section through the base of the anterior tusk 

 above the socket exhibits the structure described in Mr. Owen's first 

 memoir ; but a section of the second tusk, also taken above the socket, 

 exhibited a very simplified modification of the labyrinthic arrangement, 

 presenting a disposition closely analogous to that at the base of the 

 teeth of the Ichthyosaurus. The apical half of the tusks has a smooth 

 and polished surface, and the pulp-cavity is continued, of small size, 

 into the centre of this part of the tooth. In the serial teeth, which 

 in other respects, except size, correspond with the preceding descrip- 

 tion of the tusks, the central pulp-cavity is more quickly obliterated, 

 but the alveoli are large, moderately deep and complete : the texture 

 of the teeth is dense and brittle. The base of each tooth is anchy- 

 losed to the bottom of its socket, as in Scomberoid and Sauroid 

 fishes ; but the Labyrinthodon j)ossesses, Mr. Owen says, a still 

 more ichthyic character in the continuation, preserved in this speci- 

 men, of a row of small teeth anterior and external to the two or three 

 larger tusks. A double row of teeth thus occasioned does not exist 

 in the maxillary bones, either superior or inferior, of any Batra- 

 chian or Saurian reptile ; in Mammalia it has been noticed only in 

 the upper jaw of the hare and rabbit, and in Fishes only in the 

 lower jaw. 



A fragment of the superior maxillary bone is also described, and 

 its chief deviation from the Crocodilian type of structure is the con- 

 tinuation of the palatal plate of the intermaxillary bone for about 

 an inch to the outer side of the base of the external plate or pro- 

 cess ; while in the Crocodiles the external wall of the intermaxil- 

 lary bone is united by the whole of its outer margin with tlie 



