No. 136.] 



45 



The Trenton limestones contain enormous quantities of organic 

 remains, which may be collected in more or less perfection wher- 

 ever the rock is exposed in ravines or cliffs. Many of them, 

 how^ever, are much broken and imperfect. This is especially true 

 of the Trilobites, of which this rock contains about a dozen 

 species, varying in size from the Trinucleus, no larger than one's 

 finger nail, to the large Isotelus or Asaphus, which sometimes 

 attains the length of a foot. 



These Trilobites, which are among the most numerous and 

 characteristic fossils of the palaeozoic rocks, and which, from 

 their peculiar appearance, so unlike any still living form, have 

 been the object of so much curiosity and study, were creatures 

 belonging to the great class of Crustacea, or animals covered, 

 insect-like, with a jointed crust or armor. Of this class the com- 

 mon crab and lobster are familiar examples, and the Limulus or 

 " Horsefoot " of the Jersey shore is another, which bears a closer 

 analogy to the trilobites. 



The various species (of which several hundred are known) 

 possess essentially the same prominent features, but differ widely 

 in the proportion and arrangement of their parts. These con- 

 sist, first, of a broad arched plate or shield covering the head, 

 not unlike the buckler of the ^'horse-foot crab." This buckler 

 is divided by several joints or " sutures " in directions which 

 vary in different genera, the most marked being those which 

 separate its lateral portions or " cheeks " from the central part. 

 Though these are not often visible, and appear in any case only 

 as faint lines running across the buckler, we very frequently find 

 specimens in which they have separated, leaving the parts 

 detached. 



On this "head" or "buckler" are often found prominences of 

 various forms, the main features being usually a central and two 

 lateral elevations ; though in some species, such as the large 

 Isotelus or Asaphus of the Trenton limestone, these are obscure, 

 and the head forms almost a single flat arch. On the lateral 

 elevations are placed the eyes, which are generally very con- 

 spicuous. In some genera, these are simply smooth prominences, 

 without perceptible markings; but in others, especially the 

 Genus Phacops, they show most distinctly a large number of 

 lenses arranged on a crescent-shaped elevation, being in fact a 

 compound eye, such as is found on many Crustacea at the present 

 day, and resembling that of such insects as the dragon-fly. This 



