io8 



HISTORICAL PALAEONTOLOGY. 



mit of the calyx, and which are composed of innumerable 

 calcareous plates or joints, and are known as the " arms. " In 

 the Cystideans, on the other hand, there are either no " arms " 

 at all, or merely short, unbranched, rudimentary arms. The 

 Cystideans are principally, and indeed nearly exclusively, 



Fig. 47. Lower Silurian Crustaceans, a, Asaphus tyrannus. Upper Llandeilo ; 6. 

 Ogygia Buchii, Upper Llandeilo ; c, Trinucleus concentricus, Caradoc ; d, Caryocaris 

 Wrigfitii, Arenig (Skiddaw Slates) ; e, Beyrichia complicata, natural size and enlarged, 

 Upper Llandeilo and Caradoc ; /, Primitia strangulata, Caradoc ; g, Head-shield of 

 Calymene Blumenbachii, var. brevicapitata, Caradoc ; h, Head-shield of Triarthrm 

 Becki (Utica Slates), United States ; i, Shield of LeperdiUa Canadensis, var. Joseph- 

 iana, of the natural size, Trenton Limestone, Canada; j, The same, viewed from the 

 front. (After Salter, M'Coy, Rupert Jones, and Dana.) 



Silurian fossils ; and though occurring in the Upper Silurian 

 in no small numbers, they are pre-eminently characteristic of 

 the Llandeilo-Caradoc period of Lower Silurian time. They 



