48 



JOHAN KIÆR. 



M.-N. Kl. 



short, deflected pleural spines, and this is undoubtedly the original 



condition. 



I refer to the text-figure 6, which show the gradual reduction of 



the pleural spines, from forms like those in StrenueUa primaeva Brøgger 



to Ellipsocephalus Hofft Barrand. 



In this respect StrenueUa strenua Billings seems to show a far 



advanced stage. 



a The spines on the rhachis appears to vary 



greatly; this condition, however, often varies most 

 strongly in closely related trilobites. 



As far as the pygidium is concerned — as 

 mentioned — the known pygidiums of StrenueUa 

 strenua Bil. var. nasnta Walcott and Salopiensis 

 CoLBOLD, are quite difterent from the two pygidiums 

 that were described b}' Moberg from Torneträsk 

 and which he referred to the StrenueUa form 

 occurring there. The pygidiums that were described 

 in the case of the two first forms, on the other 

 hand, entirely agree in all respects; they remind 

 one somewhat of the pygidium of the Middle 

 Camerian Agraulos ceticcphalus Barrand and 

 differ still more from the pygidiums that are marked- 

 ly extended in breadth in the Middle Cambrian 

 EUipsocephalus forms ^. Of these pygidiums, that 

 described by Moberg should probably be considered 

 to be the most primitive. 



All of these pygidiums have a very small size and 

 indicate a great number of segments, which parti- 

 cularly in the posterior regions of the thorax may 

 be considered to have been small and markedly 

 decreasing in breadth. 



It is especialW this difference in the form of 

 the pygidium that might justify a separation of the 

 Scandinavian species into a special genus. However, 

 it seems best to defer this until fortunate discoveries 



may enable us to obtain a more complete knowledge of the structure of 



these old forms than we at present possess. 



Fig. 6. Thoracic segments 

 of a StrenueUa primaeva 

 Bkøgger b Ellipsocephalus 

 jVordenskjöldi Lnrs. c Ellip- 

 socephalus mutiais Lnks. d 

 Ellipsocephalus polytomus 

 Lnrs. e Ellipsocephalus Hoffi 

 Bar. Showing the gradual 

 reduction of the pleural spi- 

 nes in these forms, a, b, d, 

 and e after specimens in the 

 pal. mus. in Kristiania, c after 

 Linnar ssoN. All figures are 

 enlarged. 



1 We must remember, however, that all these pygidiums were found isolated ; there is 

 however, an extreme probability that they belong to the said StrenueUa forms. 



