174 STUDIES IN EVOLUTION 



3 ; from the Lower Helderberg group, Albany county, New 

 York. 4 Several of these remarkable larvae have been found 

 perfectly silicified in a limestone from which they have been 

 freed by etching. In general form they resemble the second lar- 

 val stage of Sao (Plate III, figure 9), but the pygidium is shorter 

 and the glabella does not expand and terminate in the ante- 

 rior margin. No eye-line is present, but the eye-lobes may be 

 seen a little within the margin. The glabella has the charac- 

 teristic number of annulations ; margin provided with a row 

 of denticles ; genal angles extended into spines ; pygidium 

 with four spines. 



The adult condition (figure 3) shows that the eyes have 

 moved inward and backward to near the neck segment. The 

 glabella has lost its annulations and is broken up into a 

 median lobe with two smaller ones on each side, while the 

 neck ring is projected into a spine. The changes noted here 

 are much more profound than in any of the preceding genera, 

 since Acidaspis is one of the most highly specialized of trilo- 

 bites in its glabellar structure and elaborate ornamentation. 

 The protaspis, too, partakes of this specialization, and although 

 the general form of the shield and the annulation of the axis 

 are as primitive as in Triarthrus, yet the characteristic spi- 

 nosity of the genus appears even at this early stage and is 

 a marked instance of acceleration of development. 



Arges consanguineus Clarke ; Plate IV, figure 4 ; from the 

 Lower Helderberg group, Albany county, New York. A 

 single larval form of this type has been found, and at first was 

 provisionally referred to Phaethonides* The recent publica- 

 tion by Clarke 14 of Arges consanguineus from the same 

 horizon and a comparison of the larva with the description 

 and with considerable additional material, renders it now 

 possible to determine definitely the relations of this interest- 

 ing form. As the main details of structure in Acidaspis and 

 Arges are so similar, the transformations undergone by the 

 larva are much alike in each case. The young Arges likewise 

 shows the same acceleration in the development of the spines 

 and surface ornamentation, and the retention of the primitive 



