and Clavicular Arch in Sauropterygia. 



149 



bones, and the arrest or development of the process of ossification in 

 the various elements of the skeleton. 



Of all these characters the last is the most difficult to value, for 

 there is some evidence tending to the inference that ossification 

 became better developed with the progress of geological time, and 

 surfaces which in Liassic types had always retained the cartilaginous 

 condition of immaturity, in Cretaceous types show the completed 

 ossification of old age. This condition is not, however, universal, 

 since JEretmosaurus rugosus has ossification perfected in a way not 

 known in other Liassic genera, and Stereosaurus platyomus of the 

 Cambridge Greensand has the vertebrae crowded together irregularly, 

 while the extremities of the short, wide, thick propodial bones remain 

 unossified. 



And it is remarkable that many Liassic species have the articular 

 faces of the vertebral centra deeply biconcave, while in many Cre- 

 taceous species those surfaces are nearly or quite flat ; in the shoulder 

 girdle nothing but continued ossification apparently is needed to con- 

 vert the Liassic Plesiosaurian into the Oolitic and Cretaceous Blasmo- 

 saurian type. JEretmosaurus is the nearest approach to this type, 

 known from the Lias. 



It thus appears as though some animals complete their embryology 

 early in life, others at intervals during life, while in most types the 

 embryonic development takes place gradually during successive 

 epochs of geological time, giving rise to classification of its stages, 

 indicated as genera, families, orders ; and therefore that the young 

 individuals of a late period of time simulate genera of an earlier age. 



The character which appears to be most important in Sauropterygia 

 as a ground for primary classification is the presence of two facets, 

 or one facet, on the side of the centrum for the articulation of the 

 cervical rib. If two are present, both facets are upon the centrum, 

 and exhibit many degrees of approximation, seen in Ehomaleosaurus, 

 Pliosaurus, Plesiosaurus, before the division becomes obliterated in 

 Murcenosaurus, Golymbosaurus, and the Cretaceous types. This con- 

 dition is of further interest, from the fact that among existing 

 Vertebrates a similarly divided articulation for the rib upon the 

 centrum is only known in the existing Urodele Amphibia. Most, if 

 not all, of the Plesiosauridae have the rib facet transversely cleft ; 

 while no Elasmosaurian is at present known in which the same 

 condition is found. So that a division may be made into groups with 

 ribs of the Y~tyP e an d I-type. The former sub-division includes 

 two extreme modifications, one with a long neck, which is well repre- 

 sented in the Lias by Plesiosaurus homalospondylus and P. dolicho- 

 deirus ; and a type in which the head becomes larger and the neck 

 shorter, represented by Bhomaleosaurus Gramptoni in the Lias and 

 Pliosaurus in the Oxford and Kimeridge Clays. 



