BRITISH FOSSIL REPTILES. 91 



more important as an indication of affinity, because it is not an 

 adaptive character. The two posterior and symmetrical pro- 

 cesses of the parietal extend outwards to abut against the tym- 

 panic and squamous bones, and give additional strength to the 

 point of resistance against which the lower jaw works. 



TJie inner surface of the median parietals is not, as might 

 have been anticipated, in immediate contact with the cerebral 

 membranes, but rests upon a symmetrical median single plate of 

 bone of a subquadrate form, with the posterior angles thickened 

 and supporting two surfaces which articulate more immediately 

 with the superincumbent parietals. The internal superficies is 

 concave, the external convex with two vascular foramina in the 

 same transverse line. This bone I take to be the interparietal, 

 overlapped by the ordinary lateral parietals, which are anchylosed 

 together. 



The temporal aperture is circumscribed by the jugal, zygo- 

 matic, and tympanic bones, and is reduced to much smaller 

 dimensions than in the Lacertians, owing in part to the greater 

 breadth of the zygomatic element of the temporal. In the 

 Marine Tortoises the whole of the temporal aperture is con- 

 cealed by a continuous ossification extended from the pa- 

 rietals and posterior frontals to the zygomatic arch. 



In the Crocodiles we find a part of this structure still remain- 

 ing in the osseous bridge which traverses longitudinally the 

 temporal fossa between the parietal and the posterior frontal 

 bones. 



In the Lacertians and Plesiosaurs the temporal fossa is single 

 on each side, but in the Ichthyosaurus we find a transitional 

 structure in the occurrence of a second distinct fossa in the 

 temporal region left between the zygomatic and tympanic 

 bones. 



In the general position and strong and immoveable condition 

 of the principal bones forming the pedicle for the articulation of 

 the lower jaw, there occurs, as might be expected, a deviation 

 from the Lacertian type, and a similarity to that higher Saurian 

 family, in which there is a similarly ponderous and well-armed 

 lower jaw. 



The tympanic or articular bone, instead of being attached only 

 by its upper extremity to the conjoined squamous and mastoid 

 elements of the temporal, is in the Ichthyosaur wedged in be- 

 tween the mastoid, squamous, and zygomatic elements, and is 

 further established in this position by the irregularly dentate 

 structure of the sutures. 



This bone, moreover, presents an unusual degree of solidity 

 and robustness in all the species of Ichthyosaurus. It thus 



