208 MANUAL OF ZOOLOGY. 



rings, and a caudal shield, or " pygidium," the rings of which 

 are more or less completely anchylosed. On the under surface 

 of the body nothing has hitherto been discovered, except the 

 " hypostome," or " labrum," which was a plate placed in front 

 of the mouth. No traces of ambulatory or natatory limbs, of 

 branchiae, or of antennae, have ever been discovered. The 

 eyes, when present, are compound, and usually sessile, but 

 are sometimes supported upon projecting processes. It has 

 generally been supposed that the body of the Trilobite occupied 

 the median lobe of the crust, commencing with the " glabella " 

 in front, and terminating with the " pygidium " behind ; whilst 

 the axial lobes protected a series of delicate respiratory feet ; 

 but this view is doubted by many authorities, and the question 

 is one which we have at present no means of deciding. Quite 

 recently, however, a specimen of a Trilobite has been discovered 

 in which it is said that the bases of the legs were distinctly 

 recognisable. The specimen in question was an Asaphus ; 

 but the great number and excellent preservation of Trilobites, 

 as a general rule, render it highly probable that in most cases 

 the limbs were destitute of a chitinous exoskeleton, and were 

 therefore incapable of being preserved in a fossil state. 



Fig. 65. Morphology of Trilobites. i. Angelina Sedgwickii; 2. Diagram of the 

 cephalic shield of a- Trilobite (after Salter). a Glabella; b b Free cheeks, bearing 

 the eyes (p 0} ; c c Fixed cheek, including the eye-lobe (d~) ; e e Facial suture. 



The cephalic shield of a typical Trilobite is more or less completely 

 semicircular (fig. 65, 2), and is composed of a central and of two lateral 

 pieces, of which the two latter may, or may not, be united together in 

 front of the former. 



The median portion is usually elevated above the remainder of the 

 cephalic shield, and is called the "glabella;" it protected the region of 

 the stomach, and is usually divided into from three to four lobes by lateral 

 grooves. At each side of the glabella, and continuous with it, is a small 

 semicircular area, called the "fixed cheek." The glabella, with the "fixed 

 cheeks," is separated from the lateral portions of the cephalic shield 

 termed the "movable," or "free cheeks," by a peculiar suture or line 



