406 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



do^Yncurved on each side. It is this three-lobed form that prompted 

 the name Trilobita for these animals. The raised middle region of 

 the head was prominent in some species and bore deep, transverse 

 grooves reflecting the segmentation. On each side of the head were 

 raised eye lobes. Across these eye lobes and around the front of the 

 head ran an impressed line, the suture line, along which the shell split 

 at the time when the animal molted. 



Some trilobites are preserved with the body stretched out hori- 

 zontally ; in others, it is rolled up so that the tail is tucked tightly in 

 beneath the head (pi. 4) . There were articulating devices between the 

 movable parts of the exoskeleton, and the outer parts of the thoracic 

 segments were beveled and could slide over one another. The animal 

 could only roll or unroll in the vertical plane, however. Its raised 

 middle region and the horizontal adjacent parts of the segments make 

 it evident that no side-to-side curving of the body in the horizontal 

 plane was possible. 



Complete exoskeletons are the exception in fossil finds. It is more 

 common to find only parts such as heads, individual thoracic seg- 

 ments, and tails, disarticulated from each other. The head itself is 

 often separated into parts along the suture line. 



Among the best-preserved trilobite shells laiown are some remark- 

 able ones from Virginia (pi. 1). After burial in lime mud (which 

 later became limestone), these shells were replaced by minutely gran- 

 ular quartz in a manner that preserved all the details with ex- 

 traordinary fidelity. When blocks of these limestones — they are 

 Ordovician in age — are placed in dilute hydrochloric acid, the lime- 

 stone is dissolved, but the replaced trilobite shells are unaffected and 

 so can be freed from the enclosing rock without damage. The shells 

 of any one species are not all of similar size, but form a graduated 

 series. This series gives a record of the animals' shell growth — which 

 took place by periodic molting — from that first formed, which was less 

 than 1 millimeter in length, onAvard. Articulated skeletons, like that 

 of Remopleurides^ are extremely rare in these particular limestones, 

 presumably because almost all the shells were dismembered as the dead 

 animals drifted about on the sea bottom. Thus, size series are usually 

 available only for individual parts of the exoskeleton; for example, 

 the part of the head between the suture lines. Such a series exhibits 

 the changes that took place in outline and convexity, as well as the 

 reduction in relative size of the spines. 



In the process of a trilobite's growth, new segments of the thorax 

 were developed in the tail portion. As they became fully formed at 

 the front edge, they were released to become freely jointed between 

 the head and tail. The number of segments thus formed is charac- 

 teristic for each trilobite species. Exoskeletons that include size 



