92 REPORT— 1839. 



affords a strong and unyielding point of resistance for the move- 

 ments of the lower jaw, — an adaptation which the size, arma- 

 ture, and violent uses of that jaw in predatory attacks rendered 

 indispensable. And as a similar structure and office of the lower 

 jaw of the Crocodile is the condition of a corresponding strength 

 and fixation of its articular pedicle, so in this part of the cranial 

 structure we find that the Ichthyosaurus resembles the Croco- 

 dilian and differs from the Lacertian types of structure. 



There is still another peculiarity of the articular pedicle of the 

 lower jaw of the Ichthyosaurus, for the intelligibility of which 

 it will be necessary to premise some observations on the struc- 

 ture of the same part in the existing oviparous Vertehrata. 



Commencing with the structure of the articular pedicle of the 

 lower jaw in the bird, we shall find that the tympanic bone is 

 connected with the upper jaw by two osseous columns, of which 

 the lower one abuts against the palatal bones, while the second 

 and superior extends to the lower angle of the superior maxillary 

 bone. It is by means of these two columns that the movements 

 of the articular pedicle are communicated to the upper man- 

 dible. 



It is with the upper of these osseous columns, which relates 

 to the Ichthyosaurian structure under consideration, that we are 

 at present concerned. Its usual appearance is that of a simple 

 osseous style, and it is described by Cuvier and other compara- 

 tive anatomists as the analogue of the jugal bone. If, however, 

 the state of this apparently simple style be looked into in the 

 embryo bird, it will be found to consist of two distinct parts, — 

 that ossification commences by two distinct centres. In the 

 Ostrich, indeed, which is one of the most reptilian of birds, the 

 two bones remain distinct to nearly the period of full growth. 



The anterior of these I regard as the true os jugale j the 

 posterior as the homologue of the os zygomaticum, otherwise 

 entirely wanting in the skull of the bird. 



Now here the important point gained in tracing the homolo- 

 gies of the lateral bones of the skull through the Saurian group 

 is the evidence of the separate existence of the squamous and 

 zygomatic elements of the temporal bone in the same cranium. 

 In the Lacertians the zygomatic, squamous and jugal bones are 

 always distinct ; but the zygomatic style extends to the proxi- 

 mal instead of to the distal end of the os tympanicum as in 

 Birds. Its position, parallel with the malar bone, is the same 

 however as in Birds; and, as in that class, the anterior ex- 

 tremity of the zygomatic bone is joined to the malar bone, and 

 is directly continued from it. 



In the Crocodilians the position of the os zygomaticum is 



