30 



Fossils of the Utica Slate. 



It corroborates in a remarkable degree the observations of M. 

 Barrande on the development of Arethusina Konincki^ and the 

 changes in Proetus venustidus; of the latter he says: • 



*'The specimen of ten segments is absolutely smaller than most of 

 those having but nine. Plate XV of the same work shows that in 

 Arethusina Konincki the size of the individual is not proportionate 

 to the number of liberated segments in the thorax." 



The following table shows the variations in Triarthrus Becki of 

 individuals having from thirteen to sixteen segments. 



Xumber of segments in 

 the Thorax 



Entire 

 Length 



Head 



Thorax 



Pygid- 

 ium 





mm. 



mm. 



mm. 



mm. 





7.00 



2.50 



3.75 



.75 





24.00 



7.50 



14.00 



2.50 



Fourteen 



7.00 



2.50 



3.75 



.75 



Fourteen 



12.50 



4.00 



7.00 



1.25 





17.00 



5.00 



10.50 



1.50 





25.50 



7.50 



15.50 



2.50 





30.00 



9.00 



18.50 



2.50 





19.00 



5.50 



11.50 



2 50 





28.00 



8.50 



17.00 



2.50 





48.00 



12.00 



31.00 



5.00 





16.50 



5.00 



10.00 



1.50 





23.00 



7.00 



14.00 



2.00 





53.00 



15.00 



33.00 



5.00 



The two pair of slight depressions, seen on the anterior part of 

 the glabella in the adult, have not been observed in any individual 

 with less than thirteen segments in the thorax. 



CoMPAEiso^' OF Parts dueikg Development. 



In comparing the two extremes of development as known to us, we 

 notice the relative proportions of the different parts of the body. In 

 the adult having sixteen thoracic segments the head is twenty-six 

 and two-thirds times larger, the thorax 528 times, and the pygidium 

 ten times larger than the same parts in the first degree of develop- 

 ment. The head is broad in proportion to the length ; the glabella 

 is rectilinear, and equal to the width of both cheeks ; the glabella 

 furrows are oblique and curved slightly backward, instead of being 



^ Silurien Systeme de la Bohem^, i, p. 26ft. 



