322 



PALEOZOIC SYSTEM OF ROCKS. 



present day (Phyllopods), their limbs were mostly thin, flat, soft, leaf- 

 like swimmers. Walcott, however, has recently shown that, in addition 

 to these (or perhaps instead of these), there were also slender-jointed 

 legs and spiral organs which were probably gills, as shown in the section, 

 Fig. 375, b. Fig. 375, a, is a complete restoration of the under side by 

 Walcott. On this view it is easy to see why the under side is never ex- 

 posed ; for the mud, in which they were entombed, would become entan- 

 gled among these leaf -like swimmers and numerous slender legs, and in 

 breaking the rock this would determine the line of fracture over the 



Fig. 375. 



Figs. 375, 376.— Silurian Trilobites: 

 calymene senaria (after Wolcott). Z'i 



Fig. 376. 



375. a, Restoration of under side of calymene; b, Section of 

 5. Calymene Blumenbachii ; a. Same in folded condition. 



smooth back, and leave the creature firmly attached by its ventral surface 

 to the lower piece. Not uncommonly Trilobites are found folded up on 

 their ventral surface, so as to bring head and tail together and form a kind 

 of ball. In such cases the Trilobite may be got out of the rocky matrix 

 complete ; but none the less are the feet completely hidden (Fig. 376 a). 



