SOUTHERN CALIFORNIA ACADEMY OF SCIENCES. 153 



the forms of vertebrate life. We look to the mollusca for evi- 

 dences of the contemporaneous age of the strata of widely sep- 

 arated localities, and the continuity of specific and generic forms 

 through countless ages. 



Any attempt to trace the animal life of a region can be 

 likened to the 'Voll-call of an army after a series of hard-fought 

 battles, when only a few scarred and crippled veterans remain to 

 answer to their names." Or rather, it must resemble an array 

 of ancient relics dug from some long forgotten field of combat, 

 when no survivor remains to tell the tale of the contest. 



Our only source of information is the Book of Nature, from 

 which time and the mutability of events have blotted out or mis- 

 placed paragraphs, pages, and in some instances entire chapters, 

 leaving the gaps to be filled by close study, analogy, inference, or 

 the imagination. 



Many of these spaces or blanks may be restored by future 

 discoveries ; we can only make the best possible use of the ma- 

 terial at hand, trusting to future writers of perhaps later gener- 

 ations to make the record more nearly complete. 



By laborious and long continued research the ancient strata 

 of the earth's crust have been explored, and have yielded fossil 

 skeletons of extinct animals which have proved to be of more 

 value to science than rare implements or vessels of bronze or 

 gold. 



On the American Continent vertebrate life was introduced 

 after the close of the Silurian Period, or at least no fossil re- 

 mains of vertebrates have been found in the rocks of that period. 

 During the Triassic Period vertebrates had advanced so far that 

 the husre Dinosaurs had attained an enormous development in 

 America, and left their fossil footprints in the rocks as their 

 record. But it was during the Cretaceous Period that reotilian 

 life attained its sfreatest development in America, when Turtles, 

 Crocodiles and Dinosaurs abounded in the open seas and estu- 

 aries of the period. The modern sharks also appeared at that 

 time. 



The reptiles were present in immense numbers and great 

 varietv, having come down from earlier periods. 



The Mosasaurs, which attained the sfreatest leneth of any 

 known saurian, appeared then, growing to a length of from sev- 

 enty to eighty feet. 



Weird and terrific reptiles roamed on cretaceous land : and 

 others sixty to seventy feet long, preyed upon the smaller habi- 

 tants of the cretaceous oceans ; and flyine reptiles with winio-s 

 expanding to a width of fifty feet navis'ated the air of the period. 



Immense turtles, with a length of thirteen feet, and a breadth 

 of fifteen feet between the tips of their extended flippers, inhab- 

 ited the shallow cretaceous seas of our present Plains Region. 



