388 Miscellaneous. 



MISCELLANEOUS. 



Prof. Owen's Lectures on Palaeontology. 



These Lectures were resumed, at the Theatre of the Government 

 School of Science, Jermyn Street, on the 15th of April. The sub- 

 ject of the E)iaHosauria vvas then commenced, and the following is 

 the conclusion of the Lecture, from the notes of the Professor : — 



The general form of the cranium of the common or typical si)ecies 

 oi Ichthyosaurus, e.g. Ich. communis or Ich. infermedius, resembles 

 that of the ordinary cetaceous Dolphins {DeJphinus tursio and Del- 

 phinus (hlphis) ; but the Ich. tenuirostris rivals the Delphinus yan- 

 geticus in the length and slenderness of the jaws. The essential 

 difference in the sea-reptile lies in the restricted size of the cerebral 

 cavity, and the vast depth and breadth of the zygomatic arches to 

 which the seeming expanse of the cranium is due ; still more in the 

 persistent individuality of the elements of those cranial bones which 

 have been blended into shigle though compound bones in the sea- 

 mammal. The Ichthyosaurus further differs in the great size of the 

 premaxillary and small size of the maxillary bones ; in the lateral 

 aspect of the nostrils, in the immense size of the orbits, and in the 

 large and numerous sclerotic plates, which latter structures give to the 

 skull of the Ichthyosaur its most striking features. 



The true affinities of the Ichthyosaur are, however, to be elucidated 

 by a deeper and more detailed comparison of the structure of the 

 skull ; and few collections now afford richer materials for pursuing 

 and illustrating such comparisons than the palseontological series in 

 the British Museum. 



The occiput in the Ichthyosaurus is well ossified, and of unusually 

 complex structure. It is formed by the basioccipital, a pair of 

 exoccipitals, a pair of paroccipitals, and by a superoccipital ; external 

 to which that surface is extended by the parietals, mastoids, tym- 

 panies, squamosals, and pterygoids. The chief peculiarity is the 

 large proportional size of the basioccipital. Its outer surface consists 

 of a large hemispheric condyle, with a plate of bone about half the 

 diameter of the condyle, extending forwards from its under and lateral 

 borders, and subsiding towards the upper border, which is impressed 

 by the rough surfaces for the exoccipitals. Part of the periphery 

 of the condyle is sometimes impressed by a groove indicative of the 

 attachment of the capsular ligament, and in some species there is a 

 vertical depression at the middle of the condyle. The fore part of 

 the basioccipital presents, in some species, a slight median emargina- 

 tion, as if for an outlet of an eustachian canal. Anteriorly the basi- 

 occipital joins the basisphenoid, laterally the paroccipitals, superiorly 

 the exoccipitals upon which the superoccipital rests. The latter is a 

 vertical, semicircular, or reniform plate of bone, uniting by its upper 

 borders to the parietals, which, with the m.astoids, form the ' occi- 

 pital crest,' or iipper boundary of that region. 



The exoccipitals are small and reniform. The paroccipitals are 



