168 THE CHICAGO ACADEMY OF SCIENCES. 



the creatures were more numerous, when alive, than the associated 

 molluscs or other forms which had but one chance to leave a 

 record of their lives because they secreted but a single shell 

 during their lifetime. 



The trilobites, with which the present Bulletin has to deal, 

 are a group of Arthropods which constitute the most primitive 

 one of the three sub-classes of the Crustacea. They have been 

 extinct since the close of Paleozoic time, and had their maximum 

 development in the earlier periods of that era. During Silurian 

 time when the trilobites here discussed were living, they had 

 already passed the period of their greatest expansion and were 

 just starting on their decline as an important element in the 

 marine faunas of the earth. The beginning of the history of 

 the trilobites is unknown, as they were already fully differentiated 

 in Cambrian time when we have the first really definite evidence 

 of life upon the earth. In all there have probably been over 

 two thousand species of trilobites described, belonging to nearly 

 two hundred genera. 



The only portion of the trilobite commonly preserved in the 

 fossil condition is the hard exoskeletal covering of the dorsal 

 surface of the body. This test or carapace is rarely more than a 

 millimeter in thickness, and is usually much thinner; it consists 

 of thin laminae of carbonaceous matter and calcium phosphate, 

 and in its original condition was more or less chitinous in char- 

 acter. In some forms the test is pierced with pores of greater 

 or less size which give to the surface a punctate appearance. In 

 many forms the surface is perfectly smooth and in others it is 

 covered with rounded tubercles of variable size or with other 

 sorts of surface ornamentation. Some forms, as will be seen 

 by the specific descriptions later, are furnished with great hollow 

 spines. In general form the carapace is usually sub-oval, 

 rounded at both ends, and almost always longer than wide, and 

 it is usually convex transversely. It is divided longitudinally 

 by two dorsal or axial furrows into three regions or lobes, a 

 character which first gave to these organisms the name trilobites. 

 The central region is called the axis and in the living organism 

 contained the principal organs, the viscera, the heart and the 

 central ganglia of the nervous system. The two lateral regions 

 are called pleura or pleural lobes. The body is also divided 

 transversely into three regions, i) the cephalon or head; 2) the 

 thorax ; and 3,) the pygidium or abdomen. Along its margin the 

 carapace does not terminate as a simple lamellae, but is turned 



