CRUSTACEA. 



I6 5 



separated from the axis by axal furrows, and usually exhibits on 

 its surface the lines which indicate the component pleurae, as 

 well as the longitudinal furrows on the faces of these. The ex- 

 tremity of the pygidium is sometimes simply rounded; but it 

 may be prolonged into a shorter or longer spine or "mucro," 

 and the ends of the pleurae may also be extended into spine- 

 like projections (fig. no). 



Until recently, the only structure which had been discovered 

 on the under surface of any Trilobite was a broad plate situated 

 in front of the mouth, and doubtless corresponding with the 

 upper lip " labrum " or " hypostome " of living Crustaceans 

 (fig. in). The form of this hypostome very closely resembles 

 that of the lip-plate of the recent 

 Apus, one of the Phyllopods. Quite 

 recently, Mr Henry Woodward has 

 described a specimen of Asaphus 

 platycephalus, in which, in addition to 

 the lip-plate, there is a jointed fila- 

 ment (fig. in,/), apparently springing 

 from a maxilla, and being the re- 

 mains of a maxillary " palpus." Mr 

 Woodward, who is one of the highest ,,. 



... ' . . Fig. TCI i. Buccal organs of 



living authorities upon the Crustacea, Asaphus piatycepiiaius. After 



concludes that there is no reason to 



doubt that Trilobites possessed an- 



tennas and antennules, mandibles, and maxillae, and foot-jaws ; 



though, with the exception of the above, no traces of these 



organs have ever been detected. 



Also recently, a specimen of the large Trilobite, Asaphus 

 platycephalus (fig. 112), has been described by Mr Billings as 

 possessing organs which this distinguished palaeontologist re- 

 gards as being the remains of legs. The structures in question 

 are in the form of eight pairs of apparently jointed appendages, 

 which correspond with the eight segments of the thorax, arising 

 near the middle line, and curving forwards. Mr Henry Wood- 

 ward corroborates the view propounded by Mr Billings, that 

 these structures are of the nature of ambulatory legs. Pro- 

 fessors Dana and Verrill, on the other hand, regard these re- 

 mains as being "the semi-calcified arches in the membrane 

 of the ventral surface, to which the foliaceous appendages or 

 legs were attached." 



Whichever view be adopted as to the nature of this specimen, 

 it seems tolerably certain that most Trilobites cannot have 

 possessed limbs which were furnished with a chitinous exo- 

 skeleton, and were thus capable of being preserved in a fossil 



