336 Prof. Owen's Description of the Lepidosiren annectens. 



spending central groove and lateral ridges in the concave articuiai' surface of 

 the lower jaw. Moreover, this double articular trochlea is not bony but car- 

 tilaginous in both the pedicle and the lower jaw. The convex or upper part 

 of the joint forms the termination of an elongated piece of cartilage continued 

 from that which represents the squamo-temporal bone, downwards and for- 

 wards, between the expanded end of the pterygoid plate above-described, 

 which forms its internal support, and an elongated plate of bone, which runs 

 parallel with the cartilaginous pedicle and adheres to its outer side. This 

 outer plate of bone may be regarded as a rudimental os tympanicum* : its 

 inferior extremity is regularly convex, and forms the outer edge of tlie man- 

 dibular articulation : above this part it becomes contracted, and, as it were, 

 twisted, with the posterior edge turned outwards, so as to render its external 

 surface concave ; it then again becomes expanded and compressed, as it as- 

 cends obliquely backwards and terminates in a rounded edge, which gradually 

 degenerates into cartilage. 



Each ramus of the lower jaw is composed of an articular or post-mandibu- 

 larf and a dentary piece :|:, as in most osseous fishes. The articular piece is an 

 elongated compressed bone, concave posteriorly where it forms the outer mar- 

 gin of tlie articular surface, and extending forwards in a groove on the outer 

 and near the lower part of the dentary piece ; it ends in a point near the sym- 

 physis. The cartilage, which forms the principal part of the articular conca- 

 vity, extends forwards on the inner side of the postmandibular bone half-way 

 towards the symphysis. The dentary elements are anchylosed to each other 

 at the symphysis, and each is deeply notched behind for the reception of the 

 apex of the postmandibular bone. Their masticating or upper surface is mo- 

 dified in correspondence with the dental surface of the opposed jaw ; three 

 trenchant ridges radiate from the symphysis, the anterior being like that in the 

 upper jaw the shortest and thickest, and with the external angle most pointed 

 and produced, adapted for piercing and tearing; the posterior one is the thin- 

 nest, and best fitted for cutting : the two anterior dentary ridges in each ramus 

 of the lower jaw work into the notches between the ridges of the jaw above. 



Behind, and nearly parallel with the os tympanicum, is placed an elongated 

 slender trihedral bone, pointed at both ends, having a slight sigmoid curva- 



♦ Tab. XXIII. fig. 4,/). f lb. q. % lb. r. 



