178 STUDIES IN EVOLUTION 



It has already been suggested* that the species described 

 by Barrande ^ under the generic name of Hydrocephalus are 

 probably the young of Paradoxides. This conclusion receives 

 further support from the undoubted young of OlencUus, a 

 related genus, which in its immature stages bears a strong 

 resemblance to Hydrocephalus. Tlie youngest examples of 

 the latter have a distinct pygidium, a well-developed cepha- 

 lon, and large eye-lobes at the sides of the glabella, as in 

 adult forms. Free-cheeks were evidently present, though not 

 generally preserved. See figures 79 and 80. 



The young of Olenellus asaphoides, described and illus- 

 trated by Ford 22 and Walcott,^^' ^^ also present a number of 

 features considerably in advance of a typical protaspis. The 

 immature characters are mainly the large size of the cephalon 

 and the distinct annulation of the axis. The post-protas- 

 pidian characters are the distinct and separate pygidium, the 

 adult position of the eyes, and the apparently well-developed 

 free-cheeks. In figure 82, after Ford,^^ the outer pair of 

 spines belongs to the free-cheeks, the other pair being formed 

 by the pleural extensions of the glabella, which were called 

 the interocular spines. See also figures 81 and 83. 



The young specimen of Ptychoparia monile Salter sp., 

 figured and noticed by Callaway, ^^ is 1.5 mm. in length, and 

 agrees, as far as can be determined without seeing the origi- 

 nal, with what is known of other species of the same genus. 

 It probably belongs to a stage later than the protaspis. 



Matthew 26 has carefully described some small cephala of 

 Ctenocephalus (^Hartella^ Matthewi and GonoeorypTie (Baili- 

 eXld) Baileyi., from the Cambrian of New Brunswick. The 

 fact of their being separate cephala, transverse in form, and 

 from 2 to 3 mm. in length, is sufficient to show that they do 

 not represent the youngest stages of these species. 



The immature examples of Agnostus, Trinucleus, Arethu- 

 sina, Paradoxides, Olenellus, Ctenocephalus, and Conocoryphe, 

 here briefly noticed, are of great interest in a study of the 

 ontogeny of the various species to which they pertain. In 

 the present paper, however, it is intended chiefly to establish 



