776 REVIEWS 



author's studies disclose, that it will be of interest briefly to summarize the 

 more noteworthy of them. Perhaps no more familiar type of an aquatic 

 carnivorous vertebrate can be suggested than the common gar-pike of Amer- 

 ica, a long, smooth body, short neck, propelling tail, guiding, loosely 

 connecting fins, slender jaws, etc. The later ichthyosaurs approach such a 

 type more closely than do or have any other air-breathing animals, in their 

 long body, short neck, extraordinary tail-fin, paddle-like limbs, long jaws, 

 large eyes, etc., and it is very evident that in the transition from 'forms not 

 unlike a common lizard in external appearance, the animals must have 

 passed through very great changes. In the Triassic forms chiefly those from 

 California, Dr. Merriam has demonstrated a progressive adaptation in 

 all these and in other characters. The locomotion of these early forms was 

 more by aid of the limbs and less by the tail, the limbs were larger, with 

 fewer bones, more elongated arm bones, the hind limbs were larger to supply 

 the deficiency of a weaker tail, their connection with the trunk was. stronger, 

 the pelvis was heavier, the connection of the vertebrae with each other was 

 more of the terrestrial kind — the vertebrae were more elongated, that isj 

 less fish-like, etc. The skull was shorter, the jaws relatively less elongated, 

 the teeth were more firmly fixed, the eyes were smaller, the ears less well 

 adapted for deep diving, and the neck was less short. But it is in the tail 

 that the most interesting progressive adaptation is seen. Every one knows 

 how remarkable was the terminal caudal fin of the late ichthyosaurs. Dr. 

 Merriam shows that the early forms were progressively modified from the 

 simple flattened tail, as in the crocodile, to a tail with a preterminal dilata- 

 tion like that of the mosasaurs having little or no downward bend ; to the 

 gradual turning downward of the distal end and the great expanse of the 

 terminal, quite fish-like caudal fin. It is only in the ribs that modifications 

 seem to have arisen not in conformity with the laws of aquatic speciahza- 

 tion. The early forms had them attached to the centrum by a single 

 head, while the laters ones are predominantly bicipital. However, the 

 writer has Httle doubt that this primitive branch of the reptihan stem began 

 with single-headed ribs, and that the acquisition of a double-headed attach- 

 ment of a kind almost peculiarly their own, has been an independently 

 acquired character. On the other hand, it seems very certain that a similar 

 mode of attachment in the neck ribs of the plesiosaurs was a primitive 

 character which has been lost in all the late forms. As the author says: 

 ' ' Not only is the stage of development of the Triassic representatives nearer 

 the stem or semi-aquatic reptilian type than in the later ones, but a definite 

 and fairly regular gradation or progressive specialization from the earliest 

 forms to the latest seems to be recognizable in many parts of the skeleton." 



