90 Dynamic Theory. 



Hemipters by the Cicada ; the Lepidopters ( Butterfly, etc. ) by the 

 Tineites, Sphinx ; the Dipters ( House fly, etc. ) by the Culex, Chirono- 

 mus, Musca, etc.; the Hymenopters (Wasp tribes) by Apiaria.* The 

 Crustaceans of the Jurassic make an advance toward the Brachyouran 

 or (short tail) true crabs, by the broadening of the front end, the 

 thorax, and shortening the abdominal region of the Macrouran or lob- 

 ster forms. 



THE CRETACEOUS PERIOD.^ 



This period takes its name from the chalk beds of Europe. Chalk is 

 pure carbonate of lime, and is shown to be composed of the shells of 

 minute Protozoans, Rhizopods, Coccoliths, Coccospheres, etc., all 

 which are carbonate of lime. Interspersed among these are the sili- 

 ceous shells of the Protophyte Algae, Desmids and Diatoms, which by 

 a chemical process aggregate themselves into nodules of flint. Chalk is 

 being thus formed in the deep sea now. Chalk covers about 800,000 

 square miles in middle Europe to a depth of 1,000 feet in places. But 

 there is none in America. The time was occupied in America in the 

 formation of other strata which geologists can identify. 



The organic life of the Cretaceous shows important advances. In 

 plants we have at least both Angiosperms and Endogens. The former 

 include in Europe the Willow, Walnut, Maple and Holly, and in 

 America most of the modern genera, Oak, Maple, Sassafras, Dogwood, 

 Beech, Poplar, Laurel, Walnut, Scyamore, Hickory, &c. A few 

 Palms have been found in Vancouver Island. 



Some of the genera of the Cretaceous existed in greater variety of 

 species than they do now. ' ' For example, there are now only two spe- 

 cies of Sassafras, one species of Plane tree, one of Liriodendron, and 

 one of Liquid Amber. These are evidently the remnants of an extinct 

 flora." (Le Conte 459.) 



Besides the Protozoans named as being found in the C'halk, and as in 

 fact furnishing the material for it, 100 species of sponges have been 

 found. The Crinoids, which gradually supplanted the Cystids, almost 

 run out in the Cretaceous. Thus Dana reports over 500 species as 

 belonging to the Palaeozoic, 75 to the Jurassic, and only 15 to the 

 Cretaceous. The free Echinoids are abundant in the Cretaceous and 

 many of them are similar to those yet found in the deep sea. 



The bivalves (Lamellibranchs) are now in excess of the Brachiopods 

 and of nearly all the modern types. 



The Teliost fishes also come to the front. There are still plenty of 

 the Ganoids and Placoids. But the Placoids (sharks) develop in the 

 new squalodont characteristics sharp, knife-like, smooth-margined, 

 lancet-shaped teeth. 



*Daiia46l. 



