326 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



confine themselves to giving them provisional 

 names, trusting that some lucky chance may some 

 day allow a more precise determination. Some of 

 these footprints, such as the Oldhamia, radiate round 

 one central point ; others, the Arenicolitce, are small 

 striated lines ranged in two files and attributed 

 to the passage of marine worms or Annelidse ; and 

 others again are formed of two parallel furrows, 

 and are doubtless the tracks of Crustacea. 



By the side of these problematic organisms, we 

 find well marked zoological forms, such as the 

 spicula of Silicious Sponges, very primitive Corals ; 

 chitinous Alcyonaries of the extinct group of 

 Graptolites ; Echinoderms belonging to the three 

 types of Crino'ids, Cystoidea, and Asterias ; numerous 

 Brachiopods, of which one family, that of the Lingulse 

 with horned shell, has passed from the lower Cam- 

 brian through the entire series of geological forma- 

 tion down to our own seas, without undergoing 

 hardly the slightest morphological modification. 

 Molluscs, though relatively few in number, are, 

 however, represented by a few species of Lamelli- 

 branchs of the Area family, and especially by 

 several genera of pelagic habitat, of the group of 

 Pteropods. Cephalopods themselves, the highest 

 type of the Mollusc class, have been recently dis- 

 covered in the Cambrian of Esthonia and of Canada 

 in the form of straight shelled Nautilidse, akin to 

 the Orthocerata (genus Volborthella). 



Finally the class of Marine Arthropods is richly 

 represented either by a few Ostracods enclosed be- 

 tween their two calcareous valves, or by some of 

 the Malacostraca, lesser neighbours of the existing 



