MONOGRAPH OF THE EXISTING CEINOIDS. 



179 



Muscular articulations are of two types — one in which the fulcral ridge runs 

 across the joint face at right angles to the dorsoventral plane and the two halves of 

 the ventral portion of the joint face on either side of the median line are alike and 

 symmetrical, known as the straight muscular articulation (part 1, fig. 31, p. 71), 

 and one in which the fulcral ridge is strongly diagonal, so that the two portions of 

 the ventral surface on either side of 

 the median line are very different in 

 size and shape,' and there is a pinnule 

 socket on the ventrolateral border of 

 the larger ventral division, known as 

 the oblique muscular articulation 

 (part 1, fig. 32, p. 71). The distal 

 ends of axillary ossicles bear two mus- 

 cular articulations side by side, the ap- 

 posed borders of which are more or 

 less cut away (figs. 1102, 1104, pi. 19. 

 and 1128, 1136, pi. 21). In character 

 these are commonly intermediate be- 

 tween straight and oblique muscular 

 articulations. In reality they should 

 be considered in the same light as 

 single oblique muscular articulations, 

 for all gradations occur between two 

 similar articulations side by side on 

 the distal end of an ossicle through 

 stages in which one of these gradually 

 diminishes in size to the single articu- 

 lation bearing a cirrus socket on its 

 outer border. 



Viewed from the dorsal side the 

 straight muscular articulation passes 

 directly across the arm at right angles 

 to its longitudinal axis (note the ar- 

 ticulation between the radial and IBrj 

 in any figure), while the oblique mus- 

 cular articulation is always more or 

 less oblique, succeeding articulations al- 

 ternating in direction, with the pin- 

 nule socket just below the more distal end (part 1, fig. 1, p. 60; all the articula- 

 tions shown in the two central arms except just at the base are oblique muscular). 



The only straight muscular articulation is that between the radial and the IBrj. 

 The other muscular articulations are all of the oblique type, and always double. 



The muscular articulations of the free undivided arms do not vary greatly in 

 the different groups, but those toward the arm bases, in the division series, and 

 especially on the distal ends of the radials, become progi*essively more and more 



-Lateral view of type specime 



METttA EnPEDATA. 



