VERTEBRATE PALEONTOLOGY IN 1908 465 



strongly expressed in the earlier than in the later forms ; that 

 is to say, the Triassic species depart less widely in structure 

 from land-reptiles than is the case with those of the Oolites and 

 Chalk. All the evidence indicates that the trend of specialisation 

 in the group has been to develop such features as are of import- 

 ance for the needs of a purely aquatic life, and to suppress 

 such structure as is of little or no value in this respect. 

 Corresponding in the role of Mesozoic nature to the cetaceans 

 of the present day, the ichthyosaurs appear to have been 

 evolved from a more or less ordinary type of reptile with 

 limbs constructed mainly, if not entirely, for terrestrial pro- 

 gression. 



Derived from this unknown crawling ancestor, the genus 

 Cymbospondylus of the Middle Trias marks a stage in which the 

 reptiles of this group had already abandoned the shore as their 

 regular habitation, but still retained sufficient structural characters 

 to give a clue to the origin of ichthyosaurs in general. Con- 

 trasted with the Triassic tyep we have, at the other end of the 

 chain, such forms as Baptanodon and Opthalmosaurus, in which 

 adaptation to a deep-water existence has been carried to such 

 a degree as to have suggested to earlier investigators that these 

 creatures were the least specialised of all reptiles. As regards 

 the date of the evolution of the group, this can scarcely have 

 been later than the early part of the Triassic, and may possibly 

 have been still earlier. The ichthyosaurs are therefore one of 

 the oldest groups of reptiles, and may be derived from one 

 of the very primitive members of the class. Except in the 

 aforesaid characters indicating adaptation to an aquatic en- 

 vironment, the ichthyosaurian skeleton is indeed of an extremely 

 primitive type ; but to what group it presents the nearest 

 affinity is, in Dr. Merriam's opinion, still uncertain. 



Before leaving the group attention may be momentarily 

 directed to a letter from Dr. W. J. Holland in Science for 

 January 31st, reaffirming the fact that the supposed toothless 

 character of Baptanodon is based on an error. 



A memoir entitled to rank on the same high platform as 

 Dr. Merriam's is one by Dr. O. P. Hay on the fossil Chelonians 

 of North America, issued as No. 75 of the quarto publications of 

 the Carnegie Institution of Washington, and comprising no less 

 than 568 pages of letterpress, accompanied by 113 plates. The 

 memoir is entitled the Fossil Turtles of North America, the term 



