118 



Biology in America 



Tlic latter are cnrions fish-liko croatnros whose remains liave 

 been i'ouiid in several parts of this t'ountry and abroad. They 

 had no jaws, and were abundantly protected with coats of 

 mail, their heavy armoring of plates and spines giving them 

 both their scientific and common names. The ostracoderms 

 have long been a knotty problem for the palaeontologists. By 

 some they are related to the king crabs, by others to the as- 

 cidians, while yet others regard them as kin to the cyclostomes. 

 But whatever their relationships may be they were inidonbt- 

 edly highly specialized forms and have no direct bearing on 

 the problem of vertebrate descent. They were at one time the 



Ostracoderms 

 From Pirsson and Schuchert 's ' ' Geology, ' ' after Koken. 



dominant forms of life in the ancient seas, for though small 

 of size their stout coats of mail evidently served as efficient 

 means of preservation. Their abundance is shown by the 

 numbers of their shells, which in some places, notably in Caith- 

 ness, Scotland, occur piled together in great masses, where 

 their decaying bodies served to bind together the shell into 

 compact masses, which today form the hard tough flagstones 

 of this region. 



The cause of this great destruction is difficult to surmise. 

 Similar instances of the present are however by no means 

 unknown. The Mississippi lowlands are annually converted 

 into numerous lakes by the spring overflow from the river. 

 Into these lakes come numerous species of fish to breed, which 



