ON THE CLASSIFICATION OF THE LABYRINTHODONTS. 151 
VI. ArcuecosauriaA, Von Meyer. Vertebral column notochordal. Occipital 
condyles unossified. 
23. Archegosaurus, Goldfuss. 
B. Centra of dorsal vertebra elongate, contracted in the middle.—Genera 24 to 31. 
VII. Hetrorurerra. Skull triangular, with produced, tapering snout. Orbits 
central. Mandibular symphysis very long, about + of the length of the skull. 
24, Lepterpeton, Hurley. 
VIII. Necrriprka. Epiotic cornua much produced. Superior and inferior pro- 
cesses of caudal vertebree dilated at the extremities and pectinate. 
25. Urocordylus, Huxley. 
26. Keraterpeton, Husley. 
IX. Aistopopa. Limbs wanting. 
27. Ophiderpeton, Hurley. 
28. Dolichosoma, Hucley. 
X. Microsauria, Dawson. Thoracic plates unknown. Ossification of limb- 
bones incomplete. Dentine nearly or altogether non-plicate; pulp-cavity large. 
29. Dendrerpeton, Owen. 
80. Hylonomus, Dawson, 
31. Hylerpeton, Owen. 
DESCRIPTION OF GENERA AND SPECIES. 
I. EUGLYPTA. 
Mastodonsaurus, Jiiger. 
Salamandroides, Jiiger. 
Labyrinthodon (part.), Owen. 
Skull (figure). Triangular, broad, sides slightly concave (in the uncompressed 
skull) near the orbits; snout obtuse. Orbits. Oval, narrowed and pointed in front, 
moderate, somewhat posterior, approximated. Palatine foramina. Large, broadest 
near the middle, approximated. External nasal foramina. Small, roundish, sepa- 
rated by a distance about equal to the interorbital space. Choane. Roundish oval, 
distant, posterior to external nasal foramina. Teeth (disposition). Premaxillary 
apparently 8 or 10 on each side, larger than maxillary; maxillary very numerous, 
small, diminishing in size behind; palato-vomerine, two or more tusks in front 
of the choana, two behind it, succeeded by a few small teeth; a row of small teeth 
internal to these, which is continued transversely across the fore part of the united 
yomers; mandibular a nearly uniform series; one or two tusks form a short inner 
row near the symphysis. Teeth (structure). Conical, pointed, externally striate, 
with a thin investment of enamel above; dentine much complicated; pulp-cavity 
with sinuous and branching extensions. Mandibular articulation. A strong internal 
articular buttress; postarticular process well developed. Cranial sculpture. Radiate 
its and grooves upon each ossification ; an oval lyra commencing in the interor- 
pital space, expanding upon the face; in the premaxillary region the two grooves 
paldenly take a paced and longitudinal direction, passing between the external 
nasal foramina; maxillary and malar grooves; on the mandible there is an alveolar 
groove and a descending angular groove, which disappears near the angle of the jaw. 
Thoracic plates. Median plate rhomboidal, with four concave borders ; lateral plates 
triangular, the postero-lateral angle being produced backwards and reflected ; outer 
surface of all three strongly and radiately sculptured. Vertebre. Centra discoidal, 
biconcave, well ossified. Ribs. Some of the ribs in the dorsal region are long, stout, 
compressed in the antero-posterior direction towards the head, curved and bicipital. 
Limbs. The osseous elements of the limbs are dilated at the ends, and contracted in 
the middle, differing from each other chiefly in size. 
