116 The Rev. W. D. Convbeare's Additional Notices 



can be traced in the ichthyosaurus : they may perhaps be covered by the ex- 

 tension of the conterminous bones. But still the general outline is preserved ; 

 for this part is not broken into as in the lacertae, but an intermediate bone (p) 

 occurs, connecting the jugal (c) and the lower part of the os quadratum (o). 

 The analogies of this part of the head with the crocodile will appear yet more 

 strongly in the following statement. 



VII. Position of the Tympanal Bone, or Os Quadratum, and Parts 



SURROUNDING THE EXTERNAL OPENING OF THE TYMPANAL CaVITY. The boHC 



analogous to the os quadratum, or that part of the temporal which articulates 

 with the lower jaw and in part surrounds the tympanal cavity, is placed quite 

 loosely in all the lacerta? (as distinguished from the crocodile) ; being attached 

 only by its upper extremity to the superior or squamoso-zygomatic branch of 

 the temporal (w) at its junction with the mastoid. In the crocodile, on the 

 contrary, it adheres firmly between this bone and all the others in its neigh- 

 bourhood, being connected with the jugal by the interposed bone (/>). In all 

 these features the position of the os quadratum in the ichthyosaurus agrees 

 entirely with that in the crocodile, and differs entirely from its position in the 

 other lacertae. The effect of this is to give to all this part of the head a con- 

 tour at first sight strikingly similar to the former, and most completely dissi- 

 milar to the latter. 



A fossa, evidently connected with the external opening of the tympanal 

 cavity, may be distinctly seen in the head represented fig. 8. PI. XVI. be- 

 tween 0, p, and n ; and, although the bones are slightly displaced, may be 

 traced in the head represented PI. XVII.* — It will be seen, on comparison, to 

 agree very nearly with that of the crocodile, being surrounded posteriorly as 

 well as anteriorly by the neighbouring bones ; whereas, in the other lacertae, 

 the posterior inclosure is formed by membranes only. The mastoid pro- 

 cess (n')j which completes the inclosure posteriorly in the Ichthyosaurus, 

 appears to be a separate bone : in the crocodile it is united to the squa- 

 moso-zygomatic bone (n) ; in the lacertas it is separate, but very differently 

 placed. 



VIII. Occipital Bones. — So far as I am able to judge from the inspection of 

 numerous specimens, the structure of the occipital part of the head agrees with 



* In this specimen the squamoso-zygomatic portion of the temporal appears to be broken off; 

 and the upper extremity of the os quadratum, together with superior and lateral occipitals, are 

 forced outwards. The mastoid process however (n') is nearly in its true place.— See the de- 

 scription of the Plates. 



