ISO INVERTEBRATE PALAEONTOLOGY 



the cephalon never seems to comprise more, or less, 

 than five. 



Of the three orders usually recognized among Trilo- 

 bites, two (Hypoparia and Opisthoparia) were well 

 developed at the lowest horizons of the Cambrian. 

 They reached well-defined acmes in the Ordovician, 

 when many new families originated before the earlier 

 ones had disappeared. The Proparia arose in full 

 differentiation in that period. During the Silurian 

 Trilobites continued to show specific abundance, but a 

 marked decline in families set in. This became more 

 marked in the Devonian, after which a few gerontic 

 types lingered to the end of the Palaeozoic. 



The Hypoparia were represented in the Cambrian by 

 two families only, both of which were virtually restricted 

 to that period. The minute, almost " larval," Agnostus- 

 series, with but two body-segments and elusively similar 

 cephala and pygidia, are locally abundant in the Middle 

 and Upper Cambrian of Wales ; the group ranged from 

 the Lowest Cambrian to the Lower Ordovician, A 

 somewhat different series of Hypoparians arose in the 

 latter period, comprising larger and curiously specialized 

 types of which Trinucleus (PL x. fig. 8) and Ampyx are 

 the best-known examples. 



Opisthoparia were the most abundant and varied of 

 Lower Palaeozoic Trilobites. No fewer than eleven 

 families occurred in the Cambrian ; of these three were 

 absolutely restricted to that stage, and the remainder 

 disappeared in the succeeding period. In the absence 

 of Graptolites, Trilobites prove well adapted for zonal 

 subdivision of the Cambrian. The spinous Mesonacidae, 

 with large cephala, numerous thoracic segments, and 

 minute pygidia, characterize the Lower Cambrian (Olen- 

 ellus stage). Nearly allied Paradoxidae, often attaining 

 very great size, are restricted to the Middle Cambrian 



