152 INVERTEBRATE PALAEONTOLOGY 



the Caradocian is closely comparable with Trimerus 

 and Homalonotus of the Upper Silurian and Devonian. 

 The Cheiruridae included the most bizarre types of 

 Trilobites. Cheirurus itself had a disproportionately 

 wide glabella, while Sphaerexochus and Staurocephalus 

 had that feature inflated to a grotesque degree. Perhaps 

 the most extraordinary member of the group was the 

 Silurian Deiphon, in which the carapace was reduced to 

 mere " scaffolding," except for the glabella, which formed 

 a bulbous and warty protuberance. The Phacopidae, 

 with compound eyes comparable with, but inferior in 

 size to, those of Aeglina, were a family hardly less 

 important than the Calymenidae. Large, relatively 

 smooth forms of Dalmanitina are fairly common in 

 the Caradocian, while Phacopidella and Dalmanites are 

 abundant in Wenlock and Ludlow beds. All of these 

 families, except the Encrinuridae, persisted into the 

 Devonian, but the Phacopidae were the only abundant 

 Proparia of that period. 



Among true Crustacea, the four great groups of 

 Branchiopoda, Ostracoda, Cirripedia and Malacostraca 

 have representatives in the Lower Palaeozoic. The 

 Branchiopods, surviving at the present day in the well- 

 known Apus, are probably the most primitive Crustacea 

 discovered, and are believed to be ancestral to all other 

 members of the class. The delicacy of their chitinous 

 coverings makes them uncommon as fossils, but it is 

 interesting to note that Protocaris, a genus closely 

 resembling Apus, occurred in the Lower Cambrian. 

 The small bivalved Ostracoda are often abundant. 

 Their first occurrence was in the Ordovician, and such 

 types as Leperditia, Beyrichia and Entomis may be 

 found thickly scattered in shales of Caradoc and Silurian 

 age. Cirripedes (Barnacles) of Lower Palaeozoic date 

 are rare and unsatisfactory. There is much uncertainty 



