igiÖ. No. lo. THE LOWER CAMBRIAN HOLMIA FAUNA. QI 



part of the head, is very wide, and bears no transverse furrows. The first 

 furrow to appear is a median longitudinal one, which is obliterated at an 

 early stage. Glabellar furrows are introduced in young stages, and in later 

 stages of development there seems to be no reduction of furrows b}- their 

 obliteration successively from the front backward, such as is seen in some 

 of the later trilobites. The glabella occupies the whole length of the crani- 

 diurn in the youngest stages known, becomes proportionately shorter during 

 some cf the early nepionic stages {pusillits stages), and becomes longer 

 again in the neanic and ephebic stages. The palpebral lobes are in gene- 

 ral ver}- much longer in young stages than in later ones, but many species 

 are primitive in this regard, and retain the long eyes at maturitv \P. rugii- 

 losus group). Most of the adult characteristics are assumed at an early 

 age, so that specimens 6—10 mm. long are often almost identical in form 

 with the adult; but certain minor features such as the lateral extension of 

 the second thoracic segment, persist well into the ephebic stages". 



We may assume that this ontogenetic evolution also reproduces some 

 features of the phylogen}-. It is however by no means eas}' to decide 

 what ma}' be determined in this direction, and what may have entered by 

 adoptation to other conditions of existence. 



The protaspis stage, that strongly reminds one of the corresponding 

 stage in Mesonacidae, might be taken as evidence in favour of descent 

 from the latter; but the similarity in this earliest stage may also be 

 -due to the fact that both families descend from a common ancestor. The 

 latter assumption is supported by the circumstance that the nepionic stages 

 are so widely different in the two families; in particular, the remarkable 

 transformation of the glabella that is observable in these stages, is so 

 difterent from that of Mesonacidae both as larvae and fully developed forms, 

 that it would appear difficult to associate this condition with a direct des- 

 cent from the latter . At any rate, a short and broad glabellar form — 

 i. e. quite different from that of Alesouacidae — was undoubtedly developed 

 at a very early stage in the line of development of Paradoxidac; indeed 

 we see that even in the lower part of the Middle Cambrian this character 

 had already gone back to one of the earliest nepionic stages. 



We may also regard as a repetition of phylogenetic development, the 

 long palpebral lobes, which at no stage exhibit the connection with the 

 angles of the frontal lobes that is characteristic of Mesonacidae, the broad 

 frontal brim, the intergenal spines, and the marked prolongation of the 

 pleural spines on the 2nd segment of the thorax. 



Of these characters, only the intergenal spines may be accepted in 

 support of Walcott's theory; as we emphasized in the case of the protaspis 



