THE PROTASPIS. 137 



ation of the pygiclium. That the thorax grows at actual expense to the pygidium is shown 

 by the proportions of this specimen. In an adult of this species the pygidium, thorax, and 

 cephalon are to each other as 9:11 : 13. In the young specimen they are as 10 : 6 : 12, 

 the pygidium being longer' in proportion both to the thorax and to the cephalon than it 

 would be in the adult. 



This conception of the breaking down of the pygidium to form the thorax will be very 

 helpful in explaining many things which have hitherto seemed anomalous. For instance, 

 it indicates that the Agnostida;, whose subequal shields in early stages have been a puzzle, 

 are really primitive forms whose pygidia do not degenerate ; likewise the Eodiscidse, which, 

 however, show within the family a tendency to free some of the segments. The annelidan 

 Mesonacidae may not be so primitive after all, and their specialized cephala may be more 

 truly indicative of their status than has previously been supposed. 



The facts of ontogeny of trilobites with both small and large pygidia do show that 

 there is a reduction of the relative size of the caudal shield during the growth-stages, and 

 therefore that the large pygidium in the protaspis is probably primitive. The same study 

 also shows that the large pygidium is made up of "coalesced segments" only to the extent 

 that they are potentially free, and not in the sense of fused segments. 



WIDTH OF THE AXIAL LOBE. 



That the narrow type of axial lobe is more primitive than the wide one has already 

 been demonstrated by the ontogeny of various species, and space need not be taken here 

 to discuss the question. Most Cambrian trilobites have narrow axial lobes even in the 

 adult so that their development does not bring this out very strikingly, though it can be 

 seen in Sao, Ptychoparia, etc., but 'in Ordovician trilobites such as Triarthrus and especially 

 Isotelus, it is a conspicuous feature. 



PRESENCE OR ABSENCE OF A "BRIM." 



That the extension of the glabella to the front of the cephalon is a primitive feature 

 is well shown by the development of Sao (Barrande, 1852, pi. 7), Ptychoparia (Beecher, 

 1895 C, pi. 8), and Parado.rides (Raymond, Bull. Mus. Comp. Zool., vol. 57, 1914), although 

 in the last genus the protaspis has a very narrow brim, the larva during the stages of intro- 

 duction of new segments a fairly wide one, and most adults a narrow one. 



The brim of Sao seems to be formed partly by new growth and partly at the expense 

 of the frontal lobe, for that lobe is proportionately shorter in the adult than in the protas- 

 pis. In Cryptolithus and probably in Harpes, Harpides, etc., the brim is quite obviously 

 new growth and has nothing to do with the vital organs. Its presence or absence may not 

 have any great significance, but when the glabella extends to the frontal margin, it certainly 

 suggests a more anterior position of certain organs. In Sao, the only trilobite in which 

 anything is known of the position of the hypostoma in the young, the posterior end is con- 

 siderably further forward in a specimen a. 5 mm. long than in one 4 mm. long, thus indi- 

 cating a backward movement of the mouth during growth, comparable to the backward move- 

 ment of the eyes. 



SEGMENTATION OF THE GLABELLA. 



The very smallest specimens of Sao show a simple, unsegmented axial lobe, and the 

 same simplicity has been noted in the young of other genera. Beecher considered this as 



