FOSSILS— THEIR NATURE AND INTERPRET ATION. IO3 

 possibility of fossil remains. lUit, except in the case of 



Fig. 19. f "G. 20. 



Fig. 19.— F.chinoid. Bothriocidaris Pahleni Schm. Ordovician. W, side view ; d = anal 



opening ; am = paired ambulacra! plates with two double rows of pores and small spines ; 



t'a = single row of interambulacral plates. B, summit region with the anal opening (a). C, 



under side, with the mouth opening. 

 Fig. 20.— Vermes, Annelida. Serpula. A, S. (Spirorbis) omphalodes Gf. Devonian. B. C, 



S. (Galeolariii) socialis Gf. Jurassic. C, cross-section of the tubes. D^ Serputn gordialis 



Schl. Cretaceous. E, S. (Rotularia) sfirulaa Lam. Tertiary. 



Crustacea, it will be observed 

 that the animals belonging to 

 these classes live mainly on land 

 and in the air, and when we bear 

 in mind that fossilization is a 

 process usually requiring water 

 for the preparation of the matrfx 

 (sand, mud, gravel, etc.), and for 

 the covering of the body with 

 the material when prepared, it is 

 evident that all land and aerial 

 animals, although possessing 

 parts capable of fossilization, 

 and living in abundance, run 

 very small chance of being 

 found in the deposits made 

 under water, in which fossils are 

 mainly preserved. Hence Crus- 



tacea, being water animals, are ^i:.'l=^^^^rtrt^^'Z. 

 preserved as fossils in con- 

 siderable numbers, while the 

 other classes of Arthropoda, 

 that is, insects, spiders, and 

 Myriapods, although occasionally found, are rare, and prob- 



FiG. 21. — Arthropod, Crustacean. 

 Calymene Bluinenbachi Bgt. 

 k = cephalic shield ; r — thora.x 



Trilobite, 

 Silurian. 



ture ; / =: border ; a = eyes ; si = frontal 

 lobe; jr_/=: lateral furrows; «/"= neck-fur- 

 row; ('/"= occipital furrow ; nr = neck-lobe; 

 or = occipital ring; >y"=: dorsal furrow; 

 r/'= marginal furrow; i-/= axis;//= plura;; 

 a, ax = axis; s', s/ =: lateral lobes of the 

 pygidum; 1-13 = the 13 thoracic segments. 



