3. THE LARVAL STAGES OF TRILOBITES * 



(PLATES III-V) 



INTRODUCTION. 



IT is now generally known that the youngest stages of 

 trilobites found as fossils are minute ovate or discoid 

 bodies, not more than one millimetre in length, in which 

 the head portion greatly predominates. Altogether they pre- 

 sent very little likeness to the adult form, to which, however, 

 they are traceable through a longer or shorter series of modi- 

 fications. 



Since Barrande 2 first demonstrated the metamorphoses of 

 trilobites, in 1849, similar observations have been made upon 

 a number of different genera by Ford, 22 Walcott, 34 ' 35> 36 Mat- 

 thew, 26 ' 27 > 28 Salter, 32 Callaway, 13 and the writer 4 ' 5 ' 7 . The 

 general facts in the ontogeny have thus become well estab- 

 lished, and the main features of the larval form are fairly 

 well understood. 



Before the recognition of the progressive transformation 

 undergone by trilobites in their development, it was the 

 custom to apply a name to each variation in the number of 

 thoracic segments and in other features of the test. The 

 most notable example of this is seen in the trilobite now 

 commonly known as Sao hirsuta Barrande. It was shown 

 by Barrande 3 that Corda 17 had given no less than ten 

 generic and eighteen specific names to different stages in the 

 growth of this species alone. 



The changes taking place in the growth of an individual 



* American Geologist, XVI, 166-197, pis. viii-x, 1895. 



