166 



ANNULOSA. 



condition. The great abundance of Trilobites as fossils, and 

 their excellent preservation, as a general rule, render it pro- 

 bable that the limbs of most were of a soft and fleshy nature. 



At the same time it is 

 very possible that some 

 forms were possessed of 

 chitinous jointed limbs, 

 as great variations exist 

 in the character of the 

 appendages even within 

 the limits of a single 

 order. The general view 

 which has up to the time 

 of this discovery been 

 held is, that the body of 

 the Trilobite occupied 

 the median lobe of the 

 crust, commencing with 

 the glabella in front, and 

 terminating with the py- 

 gidium behind, whilst the 

 axial lobes protected a 

 series of delicate, mem- 

 branous respiratory feet. 

 It is supposed, however, 

 by Mr Woodward, that 

 the branchiae were borne 

 on the under surface of 

 the caudal shield. 

 As regards the systematic position of the Trilobites, they 

 have very generally been placed in the neighbourhood of the 

 PhyUopoda, or of the much higher order of the Isopoda, They 

 have been placed near the Phyllopods chiefly from the posses- 

 sion of numerous (not definite) body-rings, from the resem- 

 blance of their hypostome to the lip-plate of the Phyllopodous 

 genus Apus, and from their supposed possession of membran- 

 ous gill-feet. The recent discoveries, however, of Messrs 

 Billings and Woodward would lead to the belief that the Tri- 

 lobites, if not actually belonging to the Isopoda, have at any 

 rate closer affinities with this order than with any other. They 

 agree with the Isopods in the possession of sessile compound 

 eyes, in having the abdominal somites coalescent, and in some- 

 times possessing the power of rolling themselves up into a 

 ball. They differ, however, from the Isopods in the very im- 

 portant character that the thoracic segments of the latter are 



Fig. \\*.Asaphusplatycetlialus, Lower Silurian. 



