LOWER PALAEOZOIC FAUNAS 151 



(Paradoxides stage) ; and more advanced Olenidae, with 

 smaller cephala and more extensive pygidia, are abundant 

 in the Upper Cambrian (Olenus stage), though ranging 

 throughout the period and into the Ordovician. Olenus, 

 Parabolinella (PI. x. fig. 6) and Angelina (PI. iii. fig. 3) 

 are the best known British genera of the family. Sao, 

 a Cambrian Olenid, is remarkable for the relatively com- 

 plete knowledge available of its post-larval ontogeny; 

 Triarthrus, an Ordovician genus, is even more note- 

 worthy on account of discovery of specimens with per- 

 fectly preserved appendages. In the Ordovician the 

 families Asaphidae and Illaenidae were perhaps the 

 most important. Asaphus and Ogygiocaris (PI. x. fig. 7) 

 are abundant in the Llandeilo stage, and by their large 

 size and multiple pygidia seem to represent late forms 

 in their particular lineages. Aeglina is a curious type 

 from this period, with compound eyes rivalling those of 

 many Hymenoptera in size and complexity. It was 

 probably adapted for life in the shadows of deep water. 

 In the Silurian period the chief Opisthoparian families 

 were the Proetidae (which survived to the Permian), 

 Lichadidae and Odontopleuridae. The grotesque, 

 spinous character of the Lic/tas-series, and the " hedge- 

 hog" qualities of Ceratocephala and Acidaspis y were 

 clearly phylogerontic features. 



The Proparia were a small group of Trilobites (chiefly 

 Ordovician and Silurian) that often attained extra- 

 ordinary abundance. The four families usually recog- 

 nized were all differentiated in the Ordovician, but 

 became much more fully represented in the following 

 period. Encrinurus is not uncommon in the Llandovery 

 beds, and persisted to the Wenlock. The Calymenidae 

 were a somewhat static group. Ordovician species of 

 Calymene were very much like the famous " Dudley 

 Locusts " of Wenlock age ; while Brongniartella from 



