THE PROTASPIS. 133 
cent. In a specimen i mm. long figured by Walcott, the pygidium is 0.15 mm. long, or 15 
per cent of the whole length. 
The development of several species of trilobites from the Middle Cambrian is known. 
Barrande (1852) described the protaspis of Sao hirsuta, Peronopsis integer, Phalacroma bib- 
ullatum, P. nudum, and Condylopyge rex. Broegger figured that of a Liostracus (Geol. For. 
Forhandl., 1875, pi. 25, figs,. 1-3) and Lindstroem (1901, p. 21) has reproduced the same. 
Matthew (Trans. Roy. Soc. Canada, vol. 5, 1888, pt. 4, pis. 1, 2) has described the pro- 
taspis of a Liostracus, Ptychoparia linnarssoni Broegger, and Solenopleura robbi Hartt. 
Beecher (1895 C, pi. 8) has figured the protaspis of Ptychoparia kingi Meek, and the writer 
that of a Paradoxides (Bull. Mus. Comp. Zool., vol. 58, No. 4, 1914, pi. 1). 
Sao, Liostracus, Ptychoparia, and Solenopleura ; all have the same sort of protaspis. In 
all, the axial lobe reaches the anterior margin and is somewhat expanded at that end; in all, 
the glabella shows but slight trace of segmentation; and in all, the pygidium occupies from 
one fifth to one fourth the total length. There is considerable variation in the width of 
the axial lobe. It is narrowest in Ptychoparia, where in the middle it is only 14 per cent 
of the whole width, and widest in Solenopleura, where it is 28 per cent. In Ptychoparia 
the pygidium of the protaspis occupies from 18 to 22 per cent of the whole length. In 
the adult it occupies 10 to 12 per cent. In Solenopleura it makes up about 26 per cent of 
the protaspis, and in the adult about 8 per cent. 
In the youngest stages of all these trilobites, the pygidium is incompletely separated 
from the cephalon. The first sign of segmentation is a transverse crack which begins to 
separate the cephalon and pygidium, and by the time this has extended across the full width 
the neck segment has become rather well defined. In this stage the animal is prepared to 
swim by means of the pygidium, and first becomes active. The coincident development of 
the free pygidium and the neck-ring strongly suggests that the dorsal longitudinal muscles 
are attached beneath the neck-furrow. 
The single protaspis of Paradoxides now known, while only 1 mm. long, is not in the 
youngest stage of development. It is like the protaspis of Olenellus in having large eyes 
on the dorsal surface and a narrow brim in front of the glabella. The glabella is nar- 
rower than in the adult. . 
The initial test of no agnostid has probably as yet been seen, as all the young now 
known show the cephalon and pygidium distinctly separated. Phalacroma bibullatum and 
P. nudum are both practically smooth and isopygous when 1.5 mm. long. P. bibullatum 
shows no axial lobe at this stage, but a wide glabella and median tubercle develop later, 
and when the glabella first appears, it extends to the anterior margin. In Peronopsis integer 
and Condylopyge rex, the axial lobe is outlined on each of the equal shields in specimens 
about 1 mm. long, but is without furrows and reaches neither anterior nor posterior margin. 
From the foregoing brief description it appears that the pygidium of the protaspis 
varies in different groups from as little as 15 per cent of the total length in the Mesonacidae 
to as much as 50 per cent in the Agnostkke; that the axial lobe varies from as little as 14 
per cent of the total width in one Ptychoparia to as much as 50 per cent in Phalacroma 
nudum; that the glabella reaches the anterior margin in the Olenida;, Solenopleurida;, and 
Phalacroma bibidlatum, while there is a brim in front of it in the Olenellida?, Paradox- 
idse, and three of the species of the Agnostidse. The decision as to which of these conditions 
are primitive may be settled quite satisfactorily by study of the ontogeny of the various 
species. 
