436 



MESOZOIC ERA— AGE OF REPTILES, 



Of these, about three fourths are beetles. The earliest of the higher 

 group of insects seem to have been introduced here, although they do 

 not become abundant until the Tertiary. 



Fishes, — It will be remembered that the Placoids of the Palaeozoic 

 were nearly all Cestracionts, or crushing-toothed sharks. The Hybo- 



Fig. 682. 



Figs. 679-682.— Jurassic Fishes— Placoids : 679. Tooth of Acrodus nobilis. 680. Hybodus reticu- 

 latus, Spine and Tooth. 681. Squatina acanthoderma. 682. Ganoid : Tetragonolepis, restored, 

 and Scales of the same. 



donts, or sharks with teeth pointed, but rounded on the edges, com- 

 menced in the Carboniferous and increased in the Triassic. Now, in 

 the Jurassic the Cestracionts continue (Fig. 679), but in diminished 

 numbers. The Hybodonts culminate (Fig. 680), and the Squalodonts, 

 or modern sharks, with lancet-shaped teeth, commence in small num- 

 bers. Rays (Fig. 681), which maybe regarded as among the highest of 

 Placoids, are found in considerable numbers in the Jurassic. 



Ganoids continue, but take on far more modern forms, and have 

 now in most cases lost the vertebrated structure of the tail-fin, thus 

 foreshadowing the Teleosts, which appear in the next period. Among 

 the most characteristic Ganoids of this period, and, in fact, of this age, 

 are the Pycnodonts, a family characterized by a broad, flat body, rhom- 



