166 The American Geologist. Soptomber, 1895 
THE LARVAL STAGES OF TRILOBITES. 
By Charles E. Beecher, New Haven, Conn. 
(Plates VIII-X.) 
CONTENTS. Page 
I. Introduction 166 
II. The protaspis 167 
III. Review of larval stages of trilobites 170 
IV. Analysis of variations in trilobite larva- 177 
T. Antiquity of tlie trilobites 181 
YI. Restoration of the protaspis 182 
VII. The crustacean nauplius 186 
Till. Summary 190 
IX. References 191 
X. Explanation of plates 193 
I. 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 por- 
tion greatly predominates. Altogether they i)resent very little 
likeness to the adult form, to which, however, they are trace- 
able through a longer or shorter series of modifications. 
Since Barrande^ first demonstrated the metamorphoses of 
trilobites, in 1849, similar observations have been made upon 
a number of different genera by Ford,22 Walcott,-^^- •^■''' '"^ Mat- 
thew,2fi. 27, 28 Salter,32 Callaway,!^ and the writer.*- •'• ' The 
general facts in the ontogeny have thus become well estab- 
lished and the main features of the larval form are fairl}' well 
understood. 
Before the recognition of the progressive transformation 
undergone by trilobites in their development, it was the cus- 
tom to apply a name to each variation in the number of tho- 
racic segments and in other features of the test. The most 
notable example of this is seen in the trilobite now commonly 
known as Sao hirsnta Barrande. It was shown by Barrande^^ 
that Corda^^ 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 are 
•chiefly : the elongation of the body through the gradual addi- 
tion of the free thoracic segments ; the translation of the eyes, 
when present: the modifications in the axis of the glabella; 
the growth of the free-cheeks ; and the final assumption of the 
mature specific characters of pygidium and ornamentation. 
In the present paper the larval stages of several species are de- 
scribed and illustrated for the first time, and a review is under- 
