26 



EVOLUTION IN THE PAST 



POD-SHRIMPS 



passed away ; but many new genera had appeared. The new 

 forms, with few exceptions, were endowed with eyesight, and 

 were more broadly developed in the region of the tail (Asaphus, 

 Trinucleus, Ceraurus, etc.). The power to roll up in wood- 

 lice fashion was also more common (Calymmene) ; but this 

 mode of meeting dangers was not suggestive of a grand 

 future. 



Numerous " pod-shrimps " were disporting in the waters, 

 and of larger size than their Cambrian ancestors (Ceratio- 

 caris). 



OSTRACODS Ostracods, some dwelling in rock-pools, others careering 

 in large companies about the deep, were in a vigorous con- 

 dition. In fact, their shells were never so varied at any other 

 time (Beyrichia, Bairdia, etc.). They were mostly minute 

 animals, but some monsters nearly an inch in length were 

 among them (Leper ditiida). In some cases in the style of 

 lampshells and bivalved molluscs hinges were developed 

 on the tiny shell ; and in some shells there was a small 

 perforation through which the antennae could be thrust 

 without disturbing the valves (Cypridina). 



Attached to rocks, shells, and seaweeds, barnacles ex- 

 hibited their peculiar structure in various stages of develop- 

 ment. These animals had early departed from strict crusta- 

 ceans, and were fast becoming an isolated group, leading 

 lives similar to those of rock-clinging molluscs. Some were 

 protected by loosely-arranged shelly plates (Lepidocoleus) ; 

 whilst others were more or less completely encased in plates 

 closely set and overlapping (Turrilepas). None of these 

 barnacles possessed stalks for clinging to their resting-places. 

 They may, indeed, have been capable of locomotion, but 

 probably they did not much indulge in this exercise. Others 

 had stalks of a rude, primitive kind (Polliceps) ; and in a few 

 forms this organ was well developed (Scalpellum). 



There is no sign of small crustaceans with affinities to 

 trilobites and king-crabs, such as had appeared in the Cam- 

 brian. But the small scorpion-like animals (Eurypterids) 

 were on the increase (Echinognathus, etc.). Sea-scorpions, 

 so-called, did not, however, make their presence greatly felt 

 in Ordovician times. Their descendants, as will be seen, 



BARNACLES 



SEA- 

 SCORPIONS 



