I030 PRINCIPLES OF STRATIGRAPHY 



delta or as playa-lake deposits without direct connection with the 

 sea. Individuals are occasionally found in marine deposits which 

 cannot be interpreted as deltas, but m such cases the remains are 

 of single individuals or are fragments. Since all known specimens 

 of this kind represent merely the exoskeleton (Clarke and Ruede- 

 mann-6), it is easy to see that they may have reached their resting 

 place as pseudoplankton. A characteristic mode of occurrence 

 of these organisms is in the black muds intercalated in torrential 

 conglomerates and sandstones. The geographic distribution, too, 

 is of such a character as to indicate a river rather than a marine 

 habitat. For further discussion of this problem see page 1043 and 

 the references there cited. . 



The Limulava are known only from the Cambric, where they 

 are found in a remarkable state of preservation in black bituminous 

 shales (sapropellutytes), together with trilobites, worms and other 

 animals suggesting a marine habitat. 



Aractmida ; Pantapoda. The spiders and scorpions are typically 

 terrestrial animals, breathing by means of trachsea. The Panta- 

 poda (Pycnogonida). or sea-spiders, however, constitute a class 

 of marine organisms, resembling in many characters the Arach- 

 nida, of which class they are often considered relatives. There is 

 also a true spider. Argyroneta aqiiatica, which leads an aquatic 

 life in fresh water. Scorpions first appear in the Siluric, where 

 they are associated with euryi)terids. from which they may have 

 been derived. Spiders are known frotu the Coal Measures, but 

 insects date from the Ordovicic graptolite slates of Sweden (Pro- 

 tocimex). 



Myriopoda and Insecta. 



These are also terrestrial tracheates, but with marine representa- 

 tives. 



Larvpe of insects also live in the sea, and a number of adult 

 insects are marine in habit, though continuing to breathe air. Many 

 insects and spiders have been met with in the open sea, far from 

 land, swimming in great numbers on the surface, while others have 

 been found creeping between rocks under water by the shore. The 

 bug Halobates comprises some fourteen species living on the sur- 

 face of the sea, running about like the fresh-water bug Hydro- 

 metra, often hundreds of miles from land. In the Upper Siluric 

 of France a primitive cockroach, Palccoblattina, has been obtained. 

 Alyriopods have been found in the Devonic, but more abundantly in 

 the Carbonic. The Palaeozoic myriopods, like the Palaeozoic insects. 



