METAZOA MYRIAPODA. 383 



puzzling problem. Here we find forms in which simple 

 distinct body segments without any trace of subdivisions 

 follow one another, each segment bearing one pair of 

 jointed appendages (PI. 941, fig. i, Palaeocampa anthrax 

 Meek and Worthen). The boundaries of these segments 

 can be determined, as each is made up of a single dorsal 

 and a single ventral plate. The number of segments is 

 small and each segment bears a pair of jointed legs which 

 are similar in structure. On the upper side are tubercles 

 which carry clusters of slender needles (see PI. 941), 

 probably used as means of defence. The head of Palae- 

 ocampa is small but distinct and composed apparently of 

 only one segment. 



The Chilopoda of to-day probably descended from 

 Palaeocampa of forms similar to it, but these Myriapods 

 are more specialized in certain important features than 

 the Diplopoda, another group of living Myriapods, which 

 in all probability descended from Archipolypoda described 

 by Scudder. 



It may be that future investigations into the pre-Cam- 

 brian or the early Palaeozoic rocks will bring to light the 

 common ancestor of the Palaeocampa-like forms, and of 

 the Archipolypoda. 



The segments of the Archipolypoda are composed of 

 one dorsal and two ventral plates, but in some of the 

 types the dorsal plate is distinctly seen to be made of two 

 plates, which indicates that one apparent segment is in 

 reality composed of two segments. 



The Diplopoda, embracing the Millipeds, of which 

 Jtilus (No. 942) is a good example, have a distinct head 

 and a plump cylindrical body (which is not divided into 

 thoracic and abdominal regions). A dorsal view (No. 942, 

 specimen on the right), exhibits a succession of similar 

 segments of nearly equal size extending from one end of 

 the body to the other, while a side view shows the unusual 

 thickness of the body and the small size and great num- 

 ber of jointed, single-clawed legs fastened near the 

 median line of the ventral surface. 



