MONOGRAPH OF THE EXISTING CRINOIDS. 



177 



In a dorsal view of a postrarlial series of ossicles synarthries may be at 

 once distinguished by their straight course across the arm taken in connection 

 with the appearance of the point of contact between the two ossicles (repre- 

 senting the ends of the longitudinal ridges on the two joint faces) exactly in 

 the median dorsal line (fig. 668, p. 329: the two elements of each of the three 

 pairs of ossicles represented are 

 united by synarthry). 



Synarthries only occur between 

 the two elements of the first brachial 

 pair and their more proximal redu- 

 plications: that is to say. the two 

 elements of each of the division 

 series consisting of two ossicles, and 

 the first two elements of division 

 series consisting of four ossicles. 



Some or all of the synarthries 

 in the postradial series in certain 

 species may be replaced by syzygies 

 (see p. 174), though the converse is 

 not true. 



The term synarthry was first 

 proposed by Minckert to cover all 

 interbrachial articulations except 

 syzygies. The synarthry as here 

 understood was originally called bi- 

 faseial articulation by Carpenter. 

 The word synarthry is preferable to 

 the term bifascial articulation on ac- 

 count of its euphemism and brevity ; 

 it is also conveniently close to the 

 word syzygy to suggest a relation- 

 ship. It has seemed permissible, 

 therefore, to adopt the word synar- 

 thry from Minckert, but at the same 

 time to redefine it so as to include 

 within its meaning a single definite 

 type of articulation. 



Cniptosi/narthf'i. 



In the slender-armed varieties 

 of Comatula Solaris, C. pecfinata, 

 and C. purpurea, where the synar- 

 thries persist instead of giving place to syzygies. as in the broad-armed 

 varieties of the same species, the apposed surfaces of the ossicles become almost 

 plane; but a close examination reveals a faintly marked median ridge, on either 

 side of which are the two large but very shallow and almost obsolete ligament 

 fossae. The borders of the ligament fossse, ordinarily narrow rims, are here 



-Lateral view op ttpe specu 



SOMETRA PritESCEN.S. 



