41 



aud sessile, are placed on the free cheek near the facial suture. They are 

 generally composed of a number of spheroidal lenses, covered by a smooth 

 cornea, which differs from the test of the rest of the body. In Phacops 

 and Dalmanites the cornea is similar to the rest of the test, and is pierced 

 by a series of pores in which the lenses lie. These lenses varj' greatly in 

 number in the different genera — from 15 to 15,000. The form of the eye 

 also varies considerably. The body is composed of a series of from 2 to 

 29 narrow rings, more or less movable one on another. Each of these 

 pleura is divided by the longitudinal grooves into a central axial portion, 

 which is convex, and lateral portions. These parts in each pleura are 

 firmly united. The pleurae may be smooth or separated by a groove or 

 ridge into an anterior articulating portion and a posterior or non- 

 articulating portion. This shows that many trilobites had the power of 

 rolling up into a ball-like shape as a protection. The extremities of the 

 pleurae are usually bent back, and may be rounded or produced into 

 spines. The tail is composed of a number of segments fused into a shield. 

 Its size varies considerably in differeut genera, and its margin may be 

 entire or spinous. It is usually trilobed. On the under surface of the 

 body, attached to the front of the head, is a lip-plate or hypostome, situated 

 in front of the mouth. For a long time no trace of limbs was discovered, 

 but during the last 25 years careful investigation has yielded interesting 

 results. One or two unrolled specimens showed evidence of pairs of 

 limbs, some of which appeared to have a masticatory function, others to be 

 walking legs, and a third set swimmerets. From 1874 to 1880 Walcott 

 made a series of interesting researches by cutting thin sections of rolled-up 

 trilobites. He obtained as results that the under surface of the body was 

 enclosed by a membrane, strengthened by calcified arches, to which the 

 appendages were attached. Attached to the head were four pairs of 

 manducatory jaws ; a series of jointed legs extended beneath the body and 

 tail, one pair to each segment. To these limbs were also attached gills. 

 The development of the trilobite has been worked out very carefully by 

 Dr. Barrande. In some genera the head only appears in the youngest 

 forms, whilst in others a rudimentary tail and one or two ribs api^ear from 

 the beginning. As the animal grew, new ribs appeared one by one. The 

 association of trilobites with undoubted marine organisms in the 

 geological strata proves them to have lived in the sea. They probably 

 lived round the coasts in moderately deep water, and swam on their backs 

 near the surface. With regard to their zoological position there has been 

 much controversy. They were described first as the skeletons of unknown 

 fish, then as molluscs, then as insects ; one observer placed them among 

 the caterpillars. Modern investigations have decided their position 

 among the Crustacea. It has been proposed to refer them to the 

 Phyllopods, and also to the Isopcds, but they appear to be most nearly 

 allied to the Merostomata, of which the Limulus, or King Crab, is the 

 only living representative. There are, however, sufficient differences to 

 warrant them being placed in an order — Tribolita, by themselves. In the 



