250 MINERALS AND GEOLOGY 



or chitonous bodies (Fig. 163) recognized as the 

 jaws of species of Errantia, were found a few 

 years ago in the Hudson River Strata and other 

 Lower Silurian rocks of Ontario by Mr. J. G. 

 Hinde. Two of these, greatly magnified, are FlG - 163 - 



i c, -t no CT i IT r Jaws of Errantia. 



shewn in figure 163. Similar bodies from the 



same strata in Ohio had been previously regarded by Grinel as the 



lower jaws of Errantia. 



SUB-KINGDOM VI. 

 ARTHROPODA. 



The Arthropoda or Articulata (as they are also called) comprise 

 a large series of animals, characterised typically by their jointed legs, 

 more or less distinctly segmented body, and bilateral symmetry. 

 They include both aquatic and terrestrial forms. Some are of great 

 palseontological interest ; but many in their relations to geology are 

 comparatively unimportant. Four classes are universally recognized. 

 These comprise: 1. Crustacea; 2. Arachnida ; 3. Myriapoda ; and 

 4. Insecta or Hexapoda. 



CRUSTACEA. 



The crustaceans are mostly aquatic types, with respiratory organs 

 (when present) in the form of branchiae. They include : barnacles, 

 crabs and lobsters, wood lice, <kc., among living forms ; and an extinct 

 group, the Trilobites, with some other extinct forms, of great geologi- 

 cal interest. By uniting some of the more closely connected orders 

 we may arrange the crustaceans, generally, under ten leading sub- 

 divisions, comprising: 1. Cirripedia ; 2. Ostracoda ; 3. Phyllopoda ; 

 4. Trilobita ; 5. Merostomata ; 6. Phyllocarida ; 1. Amphipoda ; 

 8. Isopoda; 9. Stomapoda ; 10 Decapoda. 



1 . Cirripedia : The cirripeds form a small group of marine animals, 

 sedentary in their adult condition, and more resembling mollusks at 

 first sight than members of the articulated series. They secrete an 

 external, many-valved, calcareous shell ; and possess a number of 

 delicate, plume-like cirrhi, capable of protrusion beyond the shell for 

 the creation of currents in the surrounding water. Some of the 



