42 EVOLUTION IN THE PAST 



on either side with fin rays. In some cases the spine was 

 continued to its end in a straight line, as with lampreys 

 (diphycercal tail) ; in other cases the end was bent upwards 

 as in the case of sharks and sturgeons (heterocercal tail). 

 Lampreys, sharks, and sturgeons, therefore, are of antique 

 fashion as regards the tail, to say nothing of other features. 

 In the Devonian Period, fish-life, it will be admitted, was 

 fairly launched on several lines of evolution : and it must 

 have been a great factor in the balance of power in the seas. 

 SEA- Among invertebrates, sea-scorpions were widely distributed. 



SCORPIONS To judge by some greatly lengthened and enlarged forms, 

 they were exceedingly prosperous in certain districts. How 

 far they were indebted to fish diet for their robust develop- 

 ment is not clear. Some of the monsters, belonging to the 

 better swimmers of the fraternity, measured nearly six feet 

 in length (Pterygotus anglicus}. The less active swimmers, 

 with fewer facilities, it may be supposed, for self-aggrandise- 

 ment, continued of more or less modest proportions. 

 TRILOBITES The decline of trilobites, which had set in during the pre- 

 ceding Period, continued, and in most regions with some 

 rapidity. Old families were dying out, or becoming seriously 

 impoverished ; and no new families were arising to fill the 

 vacancies. Desperate efforts were being made in some 

 quarters to keep pace with the times ; and the body armour 

 was assuming florid and fantastic characters (Terataspis 

 grandis, etc.). But it was all to no purpose. There was no 

 true energy behind it : and one is reminded of the wildness of 

 Gothic architecture as it passed to its decline. The soundest 

 family on the scene appears to have been one dating from the 

 Ordovician Period, consisting of small creatures with well- 

 developed heads, and clad in armour of simple character 

 (Proetus'). This family was as yet far from extinction. 

 SHRIMP-LIKE Pod-shrimps at no time chosen vessels for the evolution 

 FORMS of crustaceans of higher type were now declining. Some 

 oddities, more or less closely related to them, were here and 

 there exhibiting themselves in round shields edged with spikes 

 (Pephricaris horripilata). Notwithstanding their horrific 

 aspect, they seem, one and all, to have disappeared after a 

 brief career. Far more important were other small forms 



