BRITISH FOSSIL REPTILES. 93 



altered ; its anterior extremity abuts against, and is confluent 

 with the squamous element of the temporal bone, whilst its op- 

 posite extremity is wedged in between the tympanic and the 

 jugal bones ; the whole of the posterior margin of the os tym- 

 panicum moreover runs parallel with, and is firmly united to 

 the OS tympanicum. 



In the Ichthyosaurus the os zygomaticum is present as a se- 

 parate bone, and resembles in its massive proportions that of 

 the Crocodile : the anterior extremity is expanded ; of this the 

 greater part is articulated with the posterior extremity of the 

 jugal bone, and the remainder with the squamous element of 

 the temporal. The opposite end of the os zygomaticum abuts, 

 as in the Bird and Crocodile, against the lower or articular ex- 

 tremity of the OS tympanicum, but without having the whole of 

 its posterior margin united with the tympanic bone, as in the 

 Crocodile; hence results that vacancy which Mr. Conybeare* 

 has termed the '' lower temporal fossa," and which he describes 

 as being bounded below by ^' another bone interposed between 

 the OS quadratum and the jugal," and considered by him '^as 

 another dismemberment of the temporal." The true homology 

 of this bone could not be appreciated whilst the squamous ele- 

 ment of the temporal bone was regarded as including also the 

 zygomatic, or as a '' squamoso-zygomatic bone" ; but when the 

 independent origin of the zygomatic bone has been determined, 

 and its modifications traced through the existing Saurian types, 

 the precise nature of the dismemberment of the temporal which 

 plays so conspicuous a part in the articulation of the lower jaw 

 of the Ichthyosaurus is at once recognisable. In the strength 

 of the zygomatic bone, and its connection with the articular in- 

 ferior end of the tympanic we perceive the Crocodilian character, 

 while in the free circumference of the zygomatic bone we find 

 them associated with Lacertian peculiarities. The Ichthyosau- 

 rus thus offers a beautiful transitional structure between the 

 two great existing modifications of the Saurian types which we 

 should in vain look for elsewhere. 



The peculiarly large orbital cavity in the Ichthyosaurus in- 

 cludes in its circumference six distinct bones : above, the ante- 

 rior, median and posterior frontals ; in front, the large lachry- 

 mal bone; below and behind, the jugal and apparently a dis- 

 tinct and peculiar posterior bone. 



In the separation and relative position of the median and an- 

 terior and posterior frontals the cranium of the Ichthyosaurus 

 accords with the usual Saurian characters ; but these bones are 



* Additional Notices, p. 114. 



