24 Percy E. Raymond — The Pygidium of the Trilohite. 



the " glabella " is less furrowed than the axial lobe of the 

 " pygidium ", a condition unusual among trilobites. In the 

 reversed position, the axial lobe of the anterior shield usually shows 

 three pairs of furrows, the normal number of glabellar furrows in 

 most trilobites. 



The only valid objection which I can see to reversing the usual 

 orientation is that in the new position the segments would overlap 

 in a direction opposite from that of all other Crustacea. That 

 difficulty appears to be a serious and fundamental one. 



The Protaspis. 



Professor Swinnerton has introduced a new point of view, but 

 one which might have been expected, by suggesting that the protaspis 

 is a specialized larva, adapted to a planktonic mode of life, and that 

 characteristics derived from its later development have no phyletic 

 significance.* This is, of course, the view which has lately been 

 held by nearly all zoologists concerning the nauplius of recent 

 Crustacea, and both views have their origin in the " annelid " 

 theory. Why, if trilobites were evolved directly from benthonic 

 annelids, did they not retain the old planktonic trochophore, instead 

 of evolving a new type, which in no way resembled the supposed 

 ancestor or its larva ? 



Swinnerton says that the larvae of the Mesonacidae are the most 

 primitive known. That may be true, as they are certainly the most 

 ancient, but he continues to overlook the fact that the very youngest 

 stage of the Mesonacid larva has not yet been found. Beecher pointed 

 this out, and roughly figured a specimen younger than any illustrated 

 by . Ford or Walcott.'' The eyes of this specimen were very 

 nearly marginal, and in the youngest protaspis probably were 

 actually on the edge. Beecher had worked over the Ford collection 

 of larval Mesonacidae, and certainly did not ignore them in making 

 his classification. As a protaspis, the larva of the Mesonacidae 

 does not differ fundamentally from that of other trilobites, but the 

 development is different, because of the absence of facial sutures. 



Irrespective of the presence or absence of eyes, the circular or 

 broadly oval form of the protaspis shows that beyond question it 

 was a member of the plankton. But the evidence seems to indicate 

 that it was not there as a secondarily adapted larva, but because its 

 ancestors occupied that habitat. The broad lateral expansions 

 (pleural lobes) present in larva and adult alike gave to the trilobites 

 a shell which was easily kept afloat, and while not adapted to rapid 

 swimming was a source of buoyancy. It has been shown above that 

 the large pygidium is primary, that is, those trilobites are most primi- 

 tive which are best adapted to keep afloat, and those most specialized 

 which become adapted, by assuming a worm-like form, to crawling 



1 Geol. Mag., Vol. LVI, 1919, p. 109. 



2 Amer. Geol., vol. xvi, 1S95, p. 175, fig. 6. 



