TRILOBITA. 359 



that the visceral cavity of the Trilobites (fig. 216, I) was 

 bounded inferiorly by a thin membrane, which is attached 

 to the lower margin of the dorsal crust all round. This 

 ventral membrane was strengthened by calcified arches, 

 which in turn supported the appendages beneath. As 

 to these latter our knowledge is not yet complete, but 

 we know that there existed a row of articulated ap- 

 pendages on each side of the middle line below. The 

 thoracic appendages seem to have been slender five-jointed 

 legs, in which the terminal segment forms a pointed claw, 



Fig. 216. Transverse section of the thorax of Calymene senaria, partially restored (after 

 0. D. Walcott). a, Dorsal crust; 5, Visceral cavity, continued laterally to the pleural 

 margins of the dorsal crust ; c, Legs, restored ; d, Epipodite ; e, Spiral gills. Enlarged 

 six times. 



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. 216, e), of the nature of gills; and branchial appen- 

 dages were probably attached to the bases of the thoracic 

 limbs as well. With regard to the appendages of the head, 

 the mouth is situated behind the hypostome, an.d is bounded 

 by four pairs of jointed manducatory appendages, the basal 

 joints of which are, partly or wholly, modified to act as jaws. 

 With regard to the development of the Trilobites, the eggs 

 have been noticed by both Barrande and Walcott. They 

 are spheroidal or cylindroidal in shape, and mostly about 



