648 



TR1L0BITES OF THE SILURIAN SYSTEM. 



figured by Martin 1 , and others may be seen in the recent publication of Professor 

 Phillips 2 . Though differing from those of the coal-measures, they appear to resemble 

 them in their small size. Some of them have the generic characters of certain Silurian 

 Trilobites, though of completely distinct species. On the whole, however, so far from 

 being characteristic of a deposit, otherwise rich in organic remains, Trilobites are com- 

 paratively of rare occurrence in the Carboniferous Limestone. 



We next descend through the Old Red Sandstone, in which we have not yet found 

 these crustaceans, though the shield-shaped heads of the singular fishes which specially 

 characterize the central member of this system, were first supposed to be the bucklers 

 of Trilobites ; a mistake by no means to be wondered at, if M. Agassiz be right in in- 

 forming us, that some of these fossils are almost connecting links between crustaceans 

 and fishes, (see p. 595.) 



It is only on reaching the Upper Silurian Strata that we enter upon the great " Tri- 

 lobitic series." In the highest zone or Ludlow Rock, we find that remarkable form 

 the Homalonotus (Konig), so distinct from any individual observed in the overlying 

 groups. This genus is peculiar to the upper formations of the system ; the Homalo- 

 notus Knightii (Konig), and H. Ludensis (nob.), being very characteristic of the 

 Ludlow Rocks, while the splendid species II. delphinocephalus (nob.), (Trimerus 

 delphinocephalus ? (Green)), occurs in the Wenlock Limestone 3 . The Calymene Blu- 

 menbachii ranges through the Ludlow and Wenlock formations, but is particularly 

 abundant only in the Lower Ludlow Rock and Wenlock Limestone, beneath which 

 we no longer detect it. The well-known and equally abundant Asaphus caudatus, 

 extends from the Lower Ludlow Rock to the base of the Wenlock formation. Both of 

 these trilobites are therefore generally characteristic of the Upper Silurian Rocks. The 

 Wenlock formation contains, however, other forms peculiar to itself, such as the Caly- 

 mene macrophthalma, C. variolaris, the remarkable new genera Acidaspis and Bumastus 

 (nobis), two species of Paradoxides, the Asaphus longicaudatus, &c. 



The Lower Silurian Rocks contain three distinct genera, the Trinucleus, Agnostus 

 and Ogygia, and several species of Asaphus, all different from those of the Upper Silu- 

 rian Rocks. In the Caradoc formation the Trinucleus is most characteristic. This 

 genus (of which 6 species are here described) pervades the Lower Silurian Rocks, oc- 

 curring not only in vast abundance in the Caradoc Sandstone, but occasionally also in 

 the underlying flags. The Entomostracites punctatus, Wahl., (Calymene ? punctata, 

 Dalm.), and the Asaphus Powisii, (nob.), seem to be peculiar to the Caradoc formation. 



Lastly, in the Llandeilo flags and associated schist, we are presented with distinct 

 forms of Asaphi, in the large Asaphus Buchii, and the still larger A. Tyrannus and others, 



1 Petrif. Derb. 2 Geology of Yorkshire, vol. ii. 



3 In describing the Dudley tract (p. 492.), I have not distinguished the Homalonotus Knightii from the H. 

 delphinocephalus ; for I had not then carefully examined the specimens, the latter species having only just been 

 discovered. (See subsequent description, p. 652.) 



