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 hirsutk, Peronopsis integer, Phalacroma bib- 

 ullatwn, 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. i, 2) has described the pro- 

 taspis of a Liostracus, Ptychoparia linnarssoni Broegger, and Soleno pleura 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. i). 



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- fur row. 



The single protaspis of Paradoxides now known, while only i 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 i 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 Agnostidae; 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 Olenidse, Solenopleuridse, and 

 Phalacroma bibullatum, while there is a brim in front of it in the Olenellidse, Paradox- 

 ids, and three of the species of the Agnostidae. The decision as to which of these conditions 

 are primitive may be settled quite satisfactorily by study of the ontogeny of the various 

 species. 



