220 MANUAL OF ZOOLOGY. 



are sometimes supported upon projecting processes. It has 

 generally been supposed that the body of the Trilobite occupied 

 the medium 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. Ac- 

 cording to Spence Bate, " the young of the Trilobites are of 

 the Nauplius form." 



B'ig. 68. — Morphology of Trilobites. i. Angelitia Sedgvjickii; 2. Diagram of the 

 cephalic shield of a Trilobite (after Salter), a Glabella; hb Free cheeks, bearing 

 the eyes ipo); cc Fixed cheek, including the eye-lobe (rf) ; e e Facial suture. 



The cephalic shield of <> typical Trilobite is more or less completely 

 semicircular (fig. 68, 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 th^ "movable" or "free cheeks" — by a peculiar suture or line 

 of division, which is known as the ' ' facial suture, " and is quite unknown 

 amongst recent Crustacea, except for a faint indication in the Limulus, and 

 more or less doubtful traces in certain other forms. The movable cheeks 

 bear the eyes, which are generally crescentic or reniform in shape, are 

 rarely pedunculated, and consist of an aggregation of facets covered by a 

 thin cornea. The facial sutures may join one another in front of the 



