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MANUAL OF ZOOLOGY. 



movable thoracic 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, com- 

 mencing 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. 



Fig. 52. Morphology of Trilobites. 1. Angelina Sedgwickii. 2. Diagram of the 

 cephalic shield of a Trilobite (after Saiter). a. Glabella ; b b. Free cheeks, 

 bearing the eyes (oo) ; cc. Fixed cheek, including the eye-lobe (d) ; ee. 

 Facial suture. 



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

 semicircular (fig. 52), 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 of 

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

 amongst recent Crustacea, except for a faint indication in the Limuhis, 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 

 glabella in which case the free cheeks will form a single piece or they 



