186 REPORT — 1841. 



fixed to the bone by anchylosis, as in the Pike and Lophius. This anatomical 

 fact militates strongly against the idea tliat the Labyrinthodon is a Saurian *. 

 No remains of the locomotive organs of the L. leptognathus have yet been 

 found. 



Labyrinthodon pachygnatlms. — The remains of this species, which have been 

 obtained, consist 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 ungual phalanges. A portion, nine and a half inches 

 long, of a right ramus of a lower jaw, in addition to the characters common to it 

 and the fragment of the lower jaw of the L. leptognathus, in the structure of 

 the angular and dentary pieces, shows that the outer wall of the alveolar process 

 is not higher than the inner, as in Frogs and Toads, the Salamanders and Me- 

 nopome, in all of which the base of the teeth is anchylosed to the inner side 

 of an external alveolar plate. The smaller serial teeth ai'e about forty in num- 

 ber, 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 symphysis, 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 the Proceedings of the Geological 

 Society, January, 1841, but a section of the second tusk, also taken above the 

 socket, shows a less complex modification of the labyrinthic arrangement, one, 

 viz. which is closely analogous to that at the base of the teeth of the Ichthyo- 

 saurus. 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 description 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 an- 

 chylosed to the bottom of its socket, as in Scomberoid and Sauroid fishes; but 

 the Labyrinthodon possesses a still more ichthyic character in the continua- 

 tion, preserved in this specimen, 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 Saurian 

 I'eptilef ; but in Batrachia it has been noticed in the lower jaw of the Ccecilia, 

 and it is not an uncommon structure in fishes. 



A fragment of the superior maxillary bone manifests a striking deviation 

 from the Crocodilian type of structure in the continuation of the palatal plate 

 of the intermaxillary bone for about an inch to the outer side of the base of 

 the external plate or process ; while in the Crocodiles the external wall of the 

 intermaxillary bone is united by the whole of its outer margin with the maxil- 

 lary, and is thence continued along the whole outer contour of the intermax- 

 illary bone. Now in the Labyrinthodon the intermaxillary bone presents the 

 same peculiar modification of the Batrachian condition of this bone as in the 

 higher organized Batrachia, the palatal process of the intermaxillary extending 

 beyond the outer plate both externally and, though in a less degree, internally, 

 where it forms part of the boundary of the anterior palatal foramen, whence 

 the outer plate rises in the form of a compressed process from a longitudinal 

 tract in the upper part of the palatal process ; it is here broken off' near its 

 margin, and the fractured surface gives the breadth of the base of the outer 



* It would be highly desirable to determine in how many of the characters above detailed 

 the Nothosaurus mirabilis, Muenster, may dexdate, like the Labyrinthodon, from the Saurian 

 type of structure : it would seem to connect the Plesiosaurus with the Labyrinthodon. 



t The successional teeth in Plesiosaurus and Not/iosaurus are sometimes so far developed 

 before they displace their predecessors as to cause the appearance of a double row. 



