184 REPORT — 1841. 



cavities ; but the palatal processes, instead of reaching to the middle line, as 

 in the Crocodiles, are very narrow, as in the Batraclda. The osseous roof of 

 the mouth is principally composed of a pair of broad and flat bones, analogous 

 to the divided vomer in Batrachia, but of much greater relative extent, ap- 

 proaching, in this respect, those of the Menopome, and defending the mouth 

 with a more extensive roof of bone than exists in any Lacertian reptile : phy- 

 siologically, therefore, the Labyrinthodon, in this part of its structure, comes 

 nearest to the Crocodile ; but the structure itself, morphologically, is essen- 

 tially Batrachian. In the Menopome and gigantic Salamander, a row of small 

 teeth extends transversely across the anterior extremity of the vomerine bones : 

 and the occurrence in the Lahyrintliodon of a similar row, consisting in each* 

 palatine bone of three median small teeth and two outer larger ones, marks 

 most strongly its Batrachian nature ; and from the outermost tooth a longi- 

 tudinal row of small and equal-sized teeth is continued backward along the 

 exterior margin of the palatine bone. The Avhole of this series of palatal 

 teeth is nearly concentric with the maxillary teeth. 



In Lacertine reptiles the examples of a row of palatal teeth are rare, and, 

 when present, it is short, and situated towards the back of the palate, upon 

 the pterygoid bones, as in the Iguana and Mosasaur. In Batrachians the most 

 common disposition of the palatal teeth is a transverse row placed at the ante- 

 rior part of the divided vomer, as in Frogs, the Menopome and gigantic Sala- 

 mander, and at the posterior part in certain toads. In the Amphiume, on the 

 contrary, the palatal teeth form a nearly longitudinal series along the outer 

 margin of the vomerine bones. The Lahyrintliodon combines both these dis- 

 positions of the palatal teeth, which are arranged transversely across the fore 

 part of the divided vomer and extend backwards along its outer margin. 

 No teeth are placed on the pterygoid bones at the back of the palate as in 

 the Saurians with palatal teeth. The posterior palatine apertures are, how- 

 ever, more completely circumscribed by bone than in most Batrachians, and 

 occupy the same relative position as in the Iguana. The posterior margin 

 only of one of the anterior apertures is exhibited in the specimen here de- 

 scribed, but from its curve I infer that the two apertures are not confluent, 

 as in the Crocodile, the Frog, or the Menopome, but that they are distant, 

 as in the Iguana. 



From the physiological condition of the nasal cavity it may be concluded 

 that the Labyrinthodon differed from the Batrachians and resembled the Sau- 

 rians in having distinct posterior nasal apertures surrounded by bone, and that 

 its mode of respiration was the same as in the higher air-breathing reptiles. 

 In the shedding and renewal of the maxillary and the transverse palatal teeth, 

 it is evident that the process took place alternately in each row, as in many 

 iishes, M-hereby the dental series was always kept in an efficient state. 



Another instructive fossil is a portion of the left ramus of an under jaw 

 of Lahyrintliodon leptognatlms from the War^-ick sandstone. It is six inches 

 long, slender and straight, the syniphysial extremity is abruptly bent in- 

 wards, and it presents almost as striking a Batrachian character as any of the 

 bones just mentioned. The angular piece is of great breadth, and is con- 

 tinued forward to near the symphysis, forming the whole of the inferior part 

 of the jaw, and extending upon the inner as far as upon the outer side of the 

 ramus, the inner plate performing the function of the detached os operculare 

 in the jaw of Saurians. The dentary bone is supported upon a deep and wide 

 groove along the upper surface of the angular piece, which also projects be- 

 yond the groove, so as to form a strong convex ridge on the external side of 

 the jaw, below the dentary piece. This character, which in the large bull-frog 

 {Rana pipiens) is confined to the posterior part of the maxillary ramus, is 

 in the Lubyrinthcdon continued to near the anterior extremity. The teeth 



