198 REPORT— 1841. 



dont Lizards of the Magnesian conglomerate have equal claims to a more 

 ancient origin. 



The questions, which the unbiased collector of evidence bearing upon the 

 fixity or mutability of species has next to resolve respecting these primaeval 

 Lizards, are, whether they appeared under the form of the low-organized spe- 

 cies which one naturalist classes with Sauria, another with Ophidia, or 

 whether they exhibit indications of having emerged, by progressive develop- 

 ment of structure, from any lower organized pre-existing group of cold- 

 blooded animals ? To these inquiries the Palaeontologist must reply, that 

 the thecodont Lizards of the Zechstein and Magnesian conglomerates combine 

 well-organized extremities, with teeth implanted in distinct sockets, instead 

 of being soldered, as in frogs, to a simple alveolar parapet ; and that there- 

 fore if they existed at the present day, they would take rank at the head of 

 the Lacertian order, and not among the families most nearly allied to the in- 

 ferior reptiles. Neither are the modifications of the skeleton of the Rhyn- 

 chosaur from the New Red Sandstone such as indicated that singular Lacer- 

 tian to have been derived from the Ophidian or Batrachian orders ; but, on 

 the contrary, they connect it more closely than any known recent species, 

 with Chelonia and Birds. 



The nearest approximation to the organization of fishes is made by the 

 Ichthyosaurus, an extinct genus which appears to have been introduced into 

 .the ancient seas subsequent to the deposition of the strata inclosing the re- 

 mains of the thecodont Lizards. The ichthyic characters of this genus of ma- 

 rine Saurians are not of a very important kind, being limited, like the modifica- 

 tions of the mammalian type in Whales, to a relationship with locomotion in 

 water, while all the modifications of the skeleton which are connected with 

 the respiratory, digestive or generative functions, are conformable with the 

 highest or Saurian type of reptiles ; such as the cranial anatomy, the large 

 size of the intermaxillary bones excepted, — the dental structure, which cor- 

 responds with that of the posterior teeth in Alligators, — the articulation of 

 the neurapophyses to the bodies of the vertebras, — the complicated pectoral 

 arch, — the sternum and complete abdominal cincture of ribs*, &c. The circle 

 of numerous imbricated sclerotic bones reaches its maximum of development 

 in the Ichthyosaurus ; but this is an exaggeration of a structure feebly sha- 

 dowed forth in some existing Saurians, and more strongly shown in Birds, 

 rather than a repetition of the simple bony sclerotic cup in Fishes. By no 

 known forms of fossil animals can we diminisli the wide interval which divides 

 the most sauroid of Fishes from an Ichthyosaurus. 



This most extraordinary Reptile is a singular compound in which Ichthyic, 

 Cetacean, and Ornithic characters are engrafted upon an essentially Saurian 

 type of structure. The Ichthyosaurus is, therefore, just such a form of animal 

 as might be expected, were specific forms unstable, to demonstrate a mutation 

 of characters or some tendency towards a progressive development into a 

 higher and more consistent type of organization. Nor is the field for testing 

 the transmutation theory less ample than the subject is favourable. We have 

 the opportunity of tracing the Ichthyosauri, generation after generation, 

 through the whole of the immense series of strata which intervene between the 

 new red sandstone and the tertiary deposits. Not only, however, is the ge- 

 neric type strictly adhered to, but the very species, which made its first abrupt 

 appearance in the lowest of the oolitic series, maintains its characters un- 

 changed and recognizable in the highest of the secondary strata. In the chalk 

 formations, for example, the genus Ichthyosaurus quits the stage of existence 



* This structure proves that the mode of generation of the Ichthyosaurus must have re- 

 sembled that of the Crocodile, and not that of the Batrachians or Fishes. 



