180 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



differentiated, assuming definite group characters, which are based upon the pro- 

 portionate reduction in the size of the muscles (figs. 1-115, pp. 6-G7, 950-1007, 

 pis. 1-4). 



In the straight mus- 

 cular articulation on the 

 distal face of the radials 

 uniting the radials with 

 the IBri there is a com- 

 p 1 e t e intergradation 

 from types with enor- 

 mously developed mus- 

 cles, as in Pentametro- 

 erbtus (figs. 114, p. 67, 

 and 1006, pi. 4), to types 

 in which the muscles are 

 almost entirely lost, as 

 in Himerometra mar- 

 tensl (fig. 41, p. 26), and 

 even quite absent, as in 

 Pontiovietra andersoni 

 (fig. 51, p. 33). 



In these last we 

 have an articulation 

 comparable with the 

 synarthry, but with the 

 fulcral ridge at right 

 angles to the median 

 dorsoventral line instead 

 of coinciding with it, 

 and with a broad furrow 

 reaching from the mid- 

 dle of the ventral border 

 nearly to the central 

 canal representing the 

 incompletely closed 

 channel through which 

 the central nerve cord 

 has reached its final po- 

 sition, which is the homo- 

 logue of the similar 

 channel seen in the bra- 

 chial syzygies. In P<m- 

 tiometra the fulcral 

 ridge is in process of disintegration, and in this type we appear to see a further step 

 in the direction of the syzygy. 



Fig. 220. — Lateral view of specimen op Crossometba helius. 



