496 



GEOLOGY 



successive genera best distinguish its successive stages, and their 

 distribution is a chief means of correlating the formations of 

 different continents, and of different provinces of the same continent, 

 as previously set forth (p. 481). They have long been extinct. 

 Figs. 359 and 372 show their three longitudinal lobes (whence their 

 name), and their three transverse divisions, head, thorax, and 

 caudal shield. The trilobites were well advanced in the scale of 

 development, possessing nearly all the anatomical systems and 



Fig. 372. Cambrian Crustacea: a, Holmia (Olenellus) broggeri Walcott, a 

 characteristic trilobite of the Lower Cambrian; 6, Olenoides curticei 

 Walcott, a Middle Cambrian trilobite; c, Ptycoparia kingi Meek, a Mid- 

 dle Cambrian trilobite; d, Agnostus interstrictus White, a Middle Cam- 

 brian trilobite; e, Aristozooe rotundata Walcott, a Cambrian phyllocarid; 

 /, Leperditia dermatoides Walcott, a Cambrian ostracode. 



physiological functions of modern crustaceans. Perhaps their 

 compound eyes, formed of many eyelets, are the best index of their 

 development. In this and succeeding periods, the number of eye- 

 lets in the trilobites' eyes ranged from a score to several thousands. 

 Some of the Cambrian trilobites, however, had no eyes, while, others 

 possessed abortive rudiments, implying that their ancestors had 

 possessed eyes. The acquisition and abortion of so important an 

 organ seems to indicate variation in the conditions of life. This 

 may mean no more than migration to deep dark waters, or the habit 

 of burrowing in the mud, where eyes became useless. The eyes 

 were often slightly raised on crescentic lobes, with the convex face 



