THE NATURAL HISTORY SURVEY. 17! 



rection more or less parallel with the posterior margin, towards 

 the lateral margins of the head, sometimes becoming coalescent 

 with the marginal furrows in front of the genal angles; these 

 furrows are known as the posterior cheek furrows. 



Most trilobites possess compound eyes, but a few genera 

 are furnished with simple eyes or occelli, while in a few others 

 organs of sight seem to be absent altogether. The compound 

 eyes vary greatly in size, sometimes being very small, while 

 in a few forms they occupy nearly the entire area of the free 

 cheek. In some genera, especially the earlier and more primi- 

 tive forms, the eyes are situated at the distal extremities of 

 elevated ridges or eye lines which extend outward from near 

 the anterior extremity of the glabella. According to their 

 structure, two kinds of compound eyes are recognized. In 

 the great majority of genera the outer or visual surface of the 

 eyes is covered with a smooth or granular integument which 

 entirely obscures, externally, their compound nature. Such 

 eyes are termed holochroal. Only a few genera, in fact only the 

 members of a single family, possess the second or schizochroal 

 type of eyes. In these the visual surface is made up of small, 

 polygonal or rounded openings for the individual facets which 

 are separated by an interstitial test. In the schizochroal eyes 

 the facets are much larger than in the holochroal and can always 

 be seen with the naked eye when they are well-preserved; 

 in number they vary from fourteen to six hundred, while in 

 the eyes of the holochroal type of structure the number of 

 facets sometimes reaches fifteen thousand. The shape of the 

 eyes is usually more or less crescentiform, with the visual sur- 

 face on the convex side and -directed laterally. In those tri- 

 lobites in which the surface of the eyes describe a complete 

 half circle, the facets are directed in such a manner as to receive 

 the light from all directions, from the front, from the rear and 

 from the sides. 



Upon the ventral side of the cephalon, the hypostome, 

 which is homologous with the upper lip or labrum of other 

 crustaceans, is a separate plate attached by an articulating 

 edge to the reflexed anterior border of the cephalic shield. 

 In different genera the hypostome is more or less variable in 

 shape, but it is frequently more or less deeply notched along 

 its posterior border. Its position upon the under side of the 

 animal is nearly horizontal. In some genera, especially among 

 those in which the anterior limbs of the facial sutures are con- 



