Ch. XXVII.] 



CARADOC AND BALA BEDS. 



563 



which ranges from the base to the summit of the formation, usu- 

 ally accompanied by Strophomena grandis (see fig. 645), and Orthis 

 vespertilio (fig. 644), with many other fossils. 



Fig. 643. 



645. 



m 



Orthis tricenaria, 



Hall. 



New York. Canada. 



| nat. size. 



Orthis vespertilio, Sotv. 



Shropshire; N. and S. 



Wales. 



i nat. size. 



Strophomena (Orthis) grandis, Sow- 



erby. f nat. size. 



Horderly, Shropshire ; also Coniston, 



Lancashire. 



Burmeister, in his work on the organization of trilobites, supposes 

 them to have swum at the surface of the water in the open sea and 

 near coasts, feeding on smaller marine animals, and to have had the 

 power of rolling themselves into a ball as a defence against injury. 

 He was also of opinion that they underwent various transformations 

 analogous to those of living crustaceans. M. Barrande, author of an 

 admirable work on the Silurian rocks of Bohemia, confirms the 

 doctrine of their metamorphosis, having traced more than twenty 

 species through different stages of growth from the young state just 

 after its escape from the egg to the adult form. He lias followed 

 some of them from a point in which they show no eyes, no joints to 

 the body, and no distinct tail, up to the complete form with the fall 



Fig. 647. 



Young individuals of Trinucleus 

 concentricus (T. ornatus, Barr). 



a. Youngest state. Natural size and 

 magnified ; the body rings not at 

 all developed. 



&. A little older. One thorax joint. 



c. Still more advanced. Three tho- 

 rax joints. The fourth, fifth, and 

 sixth segments are successively 

 produced, probably each time the 

 animal moulted its crust. 



Trinucleus concentricus, Eaton. 



Syn. T. Caractaci, Murch. 



jST. Ireland ; Wales ; Shropshire ; 



N. America : Bohemia. 



number of segments. This change is brought about before the 

 animal has attained a tenth part of its full dimensions, and hence 

 such minute and delicate specimens are rarely met with. Some of 



