142 STUDIES IN EVOLUTION 



eight (?) to twenty-six free segments ; rarely capable of rolling 

 up. Pygidium frequently small; margin entire or spinose. 



Principally Cambrian, but extending into the Ordovician. 



Including the genus Olenus Dalrnan as the type, and the fol- 

 lowing genera and sub-genera, which should doubtless fall into 

 several sub-family or even family groups: Acerocare Angelin, 

 Acrocephalites Wallerius, Agraulus Corda, Angelina Salter, 

 Anomocare Angelin, Anopolenus Salter, Asaphelina Bergeron, 

 Bavarilla Barrande, Bergeronia Matthew, Boeckia Brogger, 

 Cer atopy ge Corda, Chariocephalus Hall, Corynexochus Angelin, 

 Crepicephalus Owen, Ctenopyge Linnarsson, Cyclognathus Lin- 

 narsson, Dikelocephalus Owen (Centropleura Angelin), Dorypyge 

 Dames, Ellipsocephalus Zenker, Elliptocephala Emmons, Euloma 

 Angelin, Eurycare Angelin, Holmia Matthew, ffydrocephalus 

 Barrande (==. young Paradoxides), Leptoplastus Angelin, Lios- 

 tracus Angelin, Loganellus Devine, Menocephalus Owen, Mesona- 

 cis Walcott, Micmacca Matthew, Neseuretus Hicks, Olenelloides 

 Peach, Olenellus Hall, Olenoides Meek, Oryctocephalus Walcott, 

 Palceopyge Salter, Parabolina Salter, Parabolinella Brogger, 

 Paradoxides Brongniart, Peltura Angelin, Plutonides Hicks, 

 Proceratopyge Wallerius, Protagraulus Matthew, Protolenus 

 Matthew, Protopeltura Brogger, Protypus Walcott, Pteroce- 

 phalia Koemer, Ptychaspis Hall, Ptychoparia Corda, Remopleu- 

 rides Portlock, Sao Barrande, Schmidtia Marcou, Solenopleura 

 Angelin, Sphceropthalmus Angelin, Telephus Barrande, Triar- 

 thrella Hall, Triarthrus Green, and Zacanthoides Walcott. 



A complete study of this extensive family of trilobites 

 would contribute much in the way of generic synonymy, and 

 bring out the characters necessary for family determination 

 and subdivision. This important work must be left for 

 future investigation. So many genera have been described 

 from separate cranidia or even pygidia as to make it impos- 

 sible to deal with all of them in a systematic manner. The 

 zeal to make the most out of the earliest known faunas has 

 led many investigators to describe and recognize imperfect 

 and poorly preserved material, and to establish genera upon 

 very tenuous characters. Therefore, without a most inti- 

 mate knowledge of all the forms, any grouping of the major- 



