522 



CRUSTACEA. 



dages, and which has been described by Dr Mickleborough and 

 Mr Walcott. In this remarkable specimen (fig. 377), all the seg- 

 ments of the thorax, as well as those of the pygidium, are pro- 

 vided with jointed limbs, while branchial filaments (b) are likewise 



present. Apart from the 

 interesting specimens 



above noticed, the con- 

 dition of the under sur- 

 face of the Trilobites, 

 with its appended struc- 

 tures, has been fully in- 

 vestigated by Mr Charles 

 D. Walcott, whose re- 

 searches have been car- 

 ried on by the method 

 of making thin transverse 

 and longitudinal sections 

 of rolled -up specimens. 

 This able observer has 

 shown that the visceral 

 cavity of the Trilobites 

 (fig. 378, b) was bounded 

 inferiorly by a thin mem- 

 brane, which was attached 

 to the lower margin of 

 the dorsal crust all round. 

 This ventral membrane 

 was supported by calci- 

 fied arches, which in turn 

 supported the appendages 

 beneath. As to these 

 latter, it is now estab- 

 lished that there existed 

 a row of articulated ap- 

 pendages on each side 

 of the middle line below 

 (fig. 379). In the genus 

 Calymene, as will be seen from the accompanying illustrations, the 

 thoracic appendages were in the form of slender, five-jointed legs, 

 in which the terminal segment forms a pointed claw, and the basal 

 segment carries a jointed appendage, regarded by Mr Walcott as 

 homologous with the " epipodite " of many recent Crustaceans. 

 On each side of the thoracic cavity there is, also, attached a row of 

 bifid spiral appendages (fig. 378, e), of the nature of gills; and 

 branchial appendages were probably attached to the bases of the 



Fig- 377- — Under surface of the body of Asaphus meg- 

 istos, from the Ordovician rocks of Ohio, with the miss- 

 ing parts restored in outline, d, and d" , Doublure or 

 infolded margin of the cephalic shield, thorax, and pygid- 

 ium respectively. All the rings of the thorax and tail 

 carry jointed limbs, and branchial filaments {b) are also 

 present. In front, the bases of the last pair of cephalic 

 appendages (maxillipedes) is shown (;«). h, The hypos- 

 tome. (After Walcott.) 



