410 ANlSrUAL REPORT SMITHSONIAN INSTITUTION, 1961 



the space for powerful muscles in the thorax and tail of this species of 

 trilobite was not so great as that in the lobster tail. 



Rigidity and strength must have been important requirements of 

 the trilobite exoskeleton, since it was the framework upon which the 

 muscular system operated. On the outside of the shell are ridges and 

 grooves, pits, tubercles, and raised lines, incorrectly called "ornament." 

 These served to make the shell rigid, as do sheet-iron corrugations. 

 In well-preserved specimens, many minute openings have been ob- 

 served at the tips of short spines and tubercles (pi. 5, fig. 2). These 

 are the ends of exoskeletal canals that led to sensory hairs or from 

 glands beneath the exoskeleton. Such canals are also to be found 

 disposed around the margins of the exoskeleton. 



The main supply of the organic particles on which trilobites fed 

 must have been close to the surface of, or within, the sediments of the 

 sea bottom. We may reasonably conclude, then, that trilobites lived 

 largely in this bottom region, swimming by means of to-and-f ro move- 

 ments of their appendages, and also walking on, and digging or raking 

 in, the bottom sediments. The antennules extended forward, explor- 

 ing the region immediately ahead, and the eyes, with their many 

 small facets (pi. 6), were well adapted to detecting movements in 

 such surroundings. 



Gradually, then, a picture of the life of these animals begins to 

 emerge from a study of their anatomy. Trilobites must have made 

 impressions in the soft mud of the ancient sea bottom as they searched 

 for food. If such impressions were later filled in by sand or silt, they 

 might be preserved as fossil casts, projecting from the underside of a 

 layer of silt or sand, now converted into rock. Just such tracks and 

 trails are found in Paleozoic rocks : one sort of trail, called Rusophy- 

 cus, is known from many continents. It is bilobed, with a prominent 

 median longitudinal ridge. On each lobe are obliquely directed ridges 

 and grooves. In one example, clear impressions are believed to be 

 those of an animal's jointed appendages. 



These trails are commensurate with trilobites. They may be shal- 

 low, or deep and pocketlike, or more or less continuous. Inward and 

 backward movements of the walking limbs of the animal could have 

 scraped out the hollows, pushing out the material in the midline be- 

 hind them. Impressions in the sides of some of the deep hollows are 

 believed to have been made by the edges of the trilobite head and by 

 spines on its thoracic segments. The trails are thus interpreted as 

 shallow excavations, or perhaps even tunnels, made by trilobites in 

 the bottom sediment as they passed through it in search of food. 

 Some of the deep pockets have been thought of as excavations made 

 for the deposit of eggs, such as the horseshoe crab Limulus makes 

 today. 



