534 Mr. Owen on species of 



truncated extremity a few lines anterior to the acetabulum, as in the ilium of the 

 Crocodiles. The most marked character in which the present fossil deviates from 

 the corresponding bone in the Crocodiles, is its extent posterior to the acetabulum : 

 this part of the bone is compressed with a thick and rounded external and inferior 

 margin ; it becomes thinner and terminates in a tine edge internally and above ; 

 is slightly convex externally and concave on the opposite side, where it is also 

 excavated by the articular surface for the sacrum. 



In the Frog, as in the other tailless Batrachia, the ilium, besides being remark- 

 able for its extreme length and slenderness anterior to the acetabulum, is also cha- 

 racterized by its mode of articulation to the vertebral column, — a transverse pro- 

 cess of a single vertebra abutting against the anterior extremity of the produced 

 ilium. In the Crocodile, on the contrary, the transverse processes of two vertebrae 

 which are thickened and expanded, are joined to a rough concave articular surface 

 occupying the inner side of the ilium opposite and a little posterior to the aceta- 

 bular cavity. In some species the two articular surfaces are separated by a slight 

 interspace ; in others they are confluent, their respective proportions being indi- 

 cated by an entering angle of the non-articulating surface of the bone. 



In the fossil we find, as in the ilium of the Crocodiles, a well-marked, rough, 

 elongated, concave articular surface divided by a corresponding but less produced 

 angle of the non-articular surface of the bone, and unquestionably destined for the 

 reception of the external extremities of at least two broad and strong transversely 

 extended sacral ribs. It is continued from the process above the acetabulum upon 

 one-half of the long posterior process : the length of this articular surface is four 

 inches ; its greatest breadth one inch three lines ; the antero-posterior diameter 

 of the acetabulum is two inches ; its vertical diameter one inch and a half. In the 

 Batrachia a broad and thin process is continued downwards and inwards below 

 the acetabular cavity, representing the pubis, and this is separated from the aceta- 

 bulum by a well-developed ridge and the concavity below it. 



In the Crocodile, where the lower part of the acetabular cavity is always com- 

 pleted by the upper extremity of the pubis, the anterior and inferior part of the 

 ilium offers an obtuse process at the posterior part of the lower boundary of the 

 acetabular cavity : the Labyrinthodon agrees with the Crocodile in this structure. 

 As the iUac bone here described was discovered in the same quarry with the two 

 fragments of the cranium and the portion of the lower jaw of the Labyrinthodon 

 pachygnathus ; and as it presents a similar combination of Batrachian and Crocodi- 

 lian characters, it may be concluded to have belonged to the same species and pos- 

 sibly to the same individual. The cranial fragments correspond in size with those 

 in the head of a Crocodile between six and seven feet in length, but the ilium 

 supports an articular cavity for the reception of the head of a femur, somewhat 



