Ch. xxvn.] 



LLANDEILO FLAGS. 



441 



of strata of this age, if not entirely confined to them ; but very numerous 

 other genera accompany these. Burmeister, in his work on the organi- 

 zation 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 has 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 full 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 aro 

 rarely met with. Some of his figures of the metamorphoses of the com- 

 mon Trinucleus are copied in the annexed wood-cuts (figs. 597, 598). 



Fig. 598. 



Young individuals of Trinucleus con- 

 centricus (T. ornatug, Barr.) 



a. Youngest state. Natural size and 

 magnified ; the body rings not at all 

 developed. 



h. A little older. One thorax joint. 



c. Still more advanced. Three thorax 

 joints. The fourth, fifth, and sixth 

 segments are successively produced, 

 probably each time the animal moult- 

 ed its crust. 



Trinucleus concentricus, Eaton. 

 Syn. T. caractaci, Murch. 



N. Ireland ; Wales ; Shropshire ; N. America ; 

 Bohemia. 



A still lower part of the Llandeilo or Bala rocks consists of a black 

 carbonaceous slate of great thickness, frequently containing sulphate of 

 alumina and sometimes, as in Dumfriesshire, beds of anthracite. It has 

 been conjectured that this carbonaceous matter may be due in great 

 measure to large quantities of imbedded animal remains, for the number 

 of Graptolites included in these slates was certainly very great. I col- 

 lected these same bodies in great numbers in Sweden and Norway in 

 1835-6, both in the higher and lower graptolitic shales of the Silurian 

 system ; and was informed by Dr. Beck of Copenhagen, that they were 

 fossil zoophytes related to the Virgularia and Pennatula, genera of which 

 the living species now inhabit mud and slimy sediment. The most emi- 

 nent naturalists still hold to this opinion. 



