56 EVOLUTION IN THE PAST 



Changes were going on in Cephalopod life. Several old 

 NAUTILOIDS genera had now disappeared ; but some nautiloids of long 

 descent, in shells more or less straight, were still in evidence. 

 These old-fashioned creatures, however, were being rapidly 

 superseded by nautiloids of later development, and shelled 

 more in the manner of the Uving nautilus. 



But the greatest vigour of cephalopod life was moving 

 AMMONOIDS on the hues disclosed by ammonoids of the last Period. 

 The transitional rod-hke forms had now quite disappeared 

 (JBactrites) ; and the field was held by a variety of the coiled 

 forms known as Goniatites. The sheUs of these animals 

 remained for the most part with smooth surfaces, but in a 

 few cases were ornamented with knobs or tubercles (Gastrio- 

 ceras). 



Efforts to render the shell more portable for crawhng 

 purposes — observable in some goniatites of the preceding 

 Period — were unrelaxed : and the shell, in consequence of 

 the continued strain, was developing somewhat complicated 

 patterns beneath the surface (Pronorites, etc.). 

 TRILOBITES No change of conditions favourable to trilobites occurred. 

 Only one small family — and that dating from Ordovician 

 times — now represented that ancient and once flourishing 

 Order (Pro'etidce). The most prominent of its members were 

 small forms, slightly modified from their ancestors, chiefly 

 in regard to the tail (Phillipsia). Surely no beings other than 

 trilobites ever underwent so many head and tail changes in the 

 course of their career. Their lateral appendages, curiously 

 enough, always remained of primitive character. 

 SEA- Neither were the sea-scorpions enjoying much better 



SCORPIONS fortune. Apparently those strange creatures — ^giant forms 

 of which had appeared in the preceding Period — were now 

 represented only by small animals. Some of these, pre- 

 ferring fresh-water Ufe, were frequenting streams and pools in 

 the forests. 



Sea-scorpions seem to have died out in North American 



waters during this Period ; but in Europe their extinction 



took place somewhat later. 



SHRIMPS Pod-shrimps were still decUning ; and long before the close 



of the Period they became extinct. Prawns, lobsters, and 



