PALEOZOIC TIME — LOWER SILURIAN. 525 



ceras crehviseptum H., besides Maclurea arctica Haugliton, a species Bear M. 

 magna of the Chazy. Moreover, the formation of thick strata of limestone 

 shows that life like that of the lower latitudes not only existed there, but 

 flourished in profusion. 



BIOLOGICAL PROGRESS. 



1. General Progress. — During the Lower Silurian era progress in animal 

 life was marvelously great. Before it closed, nearly all the grander divisions 

 of marine invertebrates were represented. And these grand divisions were 

 displayed under nearly all their subdivisions. The Actinozoans were repre- 

 sented by Alcyonoids and Madreporids, as well as by Cyathophylloids ; La- 

 mellibranchs, by Monomyaries, related to the modern Avicula and Pecten ; 

 Heteromyaries, related to Modiola and Mytilus ; Dimyaries, both of the 

 Integripallial section related to Area and Nucula, and of the Sinupallial 

 section related to Cypricardia and Tellina; Pteropods, by more types and 

 much larger species than now exist; Gastropods, by the species of the 

 Trochus and Pleurotomaria types; Trilobites, by many new genera; and in 

 addition there were Exirypterids of large size. Besides all these, there were 

 Fishes, the hrst of Vertebrates. 



The chief divisions of marine Invertebrates supposed to be absent are : 

 Crustaceans above Entomostracans, that is, the typical Tetradecapods and 

 Decapods ; the Dibranchs, or Squids and Cuttles, among Cephalopods ; the 

 Echinoids among Echinoderms, and the Actinoids, or modern type of 

 Corals, among the Actinozoans. The exhibition of marine Invertebrates 

 was, therefore, very wide in range and far advanced in grade. There was 

 diversity enough to have afforded material for quite a full work on Inverte- 

 brate zoology. 



But, in addition to life in the waters, there was already life over the land, 

 and life, also, that could fly, and so bring the air above the land into new 

 service. The water-margins and moist places of the growing continents 

 were green with acrogenous plants that gave promise of future forests. 

 Insects, as the one specimen reported proves, were common almost every- 

 where. Hemipters are the so-called "Bugs" and Aphides. They are 

 incomplete in metamorphosis, like other low-grade Insects, and, therefore, 

 are a kind that might be among the earliest in geological time ; but until 

 the discovery in 1892, no fossil Paleozoic species had been reported. It has 

 already been remarked that terrestrial animal species rarely become fossil- 

 ized ; among the rarer of these are Insects, and of the rarest are Myriapods 

 and Spiders, and those Insects that do not frequent water-margins. Myria- 

 pods were probably part of the terrestrial population, and perhaps, but less 

 probably, Spiders. 



2. Culmination of the types of Graptolites, Cystoids, Pteropods, Trilobites, 

 and Ostracoids. — The Graptolite, Cystoid, Pteropod, Trilobite, and Ostracoid 

 types appear to have reached, in the Lower Silurian era, and passed, their 

 time of highest display. 



