MANUAL OF ZOOLOGY. 



salt lakes in both hemispheres, being especially abundant in 

 Great Salt Lake in Utah. 



ORDER III. TRILOBITA. This order is entirely extinct, none 

 of its members having survived the close of the Palaeozoic 

 period. It is probable that the Trilobites should be placed 

 near the Phyllopoda; but their exact position is uncertain, as, 

 with one exception, no traces of any appendages of any kind, 

 except the labrum, have hitherto been discovered in any Tri- 

 lobite. 



The body of a Trilobite (fig. 89) was covered with a " crust," 

 or exoskeleton, which shows more or less markedly a division 

 into three longitudinal lobes, from the presence of which the 

 name of the order is derived. The shell is composed of a 

 cephalic shield, a certain number of free and movable thoracic 

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



Fig. 89. 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 (o a) ; c c Fixed cheek, including the eye-lobe (d~) ; e e Facial suture. 



are more or less completely anchylosed. On the under surface 

 of the body nothing has hitherto been certainly determined, 

 except the " hypostome," or " labrum," which was a plate placed 

 in front of the mouth. No traces of branchiae, or of antennae, 

 have ever been discovered. The eyes, when present, are com- 

 pound, and truly 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. Pro- 



