142 STUDIES IN EVOLUTION 



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

 up. Fygidium frequently small ; margin entire or spinose. 



Principally Cambrian, but extending into the Ordovician. 



Including the genus Olenus Dalman 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, 

 Ajiomocare Angelin, Anopolenus Salter, Asaphelina Bergeron, 

 Bavarilla Barrande, Bergeronia Matthew, Boeckia Brogger, 

 Ceratopyge Corda, Chariocephalus Hall, Corynexochus Angelin, 

 Crepicephalus Owen, Ctenojiyge Linnarsson, Cyclognathus Lin- 

 narsson, D'lkeloceplialus Owen {Centropleura Angelin), Dorypyge 

 Dames, Ellip>socephalus Zenker, ElUptocephala Emmons, Euloma 

 Angelin, Eurycare Angelin, Holvvia Matthew, Hydrocephalus 

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

 tracus Angelin, LoganelUis Devine, llenocephalus Owen, Mesona- 

 cis Walcott, Micmacca Matthew, Neseuretus Hicks, Olenelloides 

 Peach, Olenellus Hall, Olenoides Meek, Oryctocephalus Walcott, 

 Palceopyge Salter, Parabolina Salter, Paraholinella Brogger, 

 Paradoxides Brongniart, Peltura Angelin, Plutonides Hicks, 

 Proceratopyge Wallerius, Protagraulus Matthew, Pmtolenns 

 Matthew, Protopeltura Brogger, Protypus Walcott, Pteroce- 

 phalia E-oemer, Ptychaspis Hall, Ptychoparia Corda, Remopleu- 

 rides Portlock, Sao Barrande, Schmtdtia Marcou, Solenoplevra 

 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- 



