146 STUDIES IN EVOLUTION 



The long list of genera in this family may be easily divided 

 into two sections, which are often recognized as of family rank. 



I. ASAPHID^E. Including the genera and sub-genera 

 Asaphus Brongniart (= Cryptonymus Eichwald), Asaphellus 

 Callaway, Asaphiscus Meek, Barrandia McCoy, Basilicus 

 Salter, Bathyurellus Billings, Bathyuriscus Meek, Bathyurus 

 Billings, Bolbocephalus Whitfield, Brachyaspis Salter, Bron- 

 teopsis W. Thompson, Dolichometopus Angelin, Geramphes 

 Clarke, Holasaphus Matthew, Homalopecten Salter, Isotelus 

 DeKay, Megalaspides Brogger, Megalaspis Angelin, Niobe 

 Angelin, Ogygia Brongniart, Ogygiopsis Walcott, Phillipsi- 

 nella Novak, Platypeltis Callaway, PtycTiopyge Angelin, and 

 Stygina Salter. 



This is a tolerably homogeneous group, although some of 

 the Cambrian forms have a sufficiently archaic expression 

 to make them seem a little out of place with genera of so 

 pronounced a family type as Asaphus, Niobe, Ptychopyge, 

 Megalaspis, and Isotelus. 



The elements of the glabella are generally quite obscure, 

 and even its limits cannot be clearly made out in late genera, 

 as Stygina and Asaphus. The segmental nature of the gla- 

 bella is clearly shown in Ogygia, Ogygiopsis, Homalopecten, 

 Asaphellus, Bronteopsis, and Bathyuriscus. 



The elements of the pygidium are obscurely marked in 

 Brachyaspis and Isotelus. Phillipsinella is a very small 

 form, and probably the young of an Asaphus. Barrandia, 

 Homalopecten, and Stygina serve as transition genera to the 

 Illsenidse. 



II. ILL^ENHXE. Including the genera and sub-genera 

 Ulcenus Dalman, ^glina Barrande (= Oyclopyge Corda), 

 Buwastus Murchison, Dysplanus Burmeister, Ectillcenus 

 Salter, Holocephalina Salter, Hydrolenus Salter, Illcenopsis 

 Salter, Hlcenurus Hall, Nileus Dalman, Octillcenus Salter, 

 Panderia Volborth, Psilocephalus Salter, Symphysurus Gold- 

 fuss, and Thaleops Conrad. 



The Illsenidse form a much more compact group than the 



