198 ACriNOMFTUA I'YI'ICA. 



ed l)y n delicate dotted line which crosses the dorsal sur- 

 face of the compound joint, the dots indicating the gajis 

 hetwceu adjacent pairs of corresponding ridges ^). 



Jn a true ligamentous articulation, on the other hand, 

 such as occurs between the outer radials of every Ante- 

 (lon ^) and of most Actinometrae •') each of the articular 

 faces is divided by a vertical ridge into two fossae which 

 give attachment to the large interarticular ligaments. Each 

 of these fossae is marked more or less distinctly by a series 

 of concentric lines like the lines of growth in a bivalve 

 shell ; and the vertical articular ridge that separates them 

 usually stands up rather prominently, especially around the 

 opening of the central canal. 



In Actinometra typica , however , these features are not 

 visible on the apposed faces of the two outer radials *) , 

 which are almost flat and cannot be said to have any dis- 

 tinct articular ridge; and although there is a median ver- 

 tical line on each face which divides it into two lateral 

 halves, yet these are not fossae and show no traces of any 

 concentric markings. Each of them is marked by a num- 

 ber of little elevations, squarish or oblong in shape, and 

 so arranged as to have their longer axes radiating out- 

 wards from the central canal. This is much more marked 

 in some cases than in others, and the result is that the 

 joint-face looks as if the radiating ridges of a syzygy were 

 interrupted at intervals. They do not , however , reach the 

 dorsal margin, or only very rarely, so that the dotted line 

 indicating a syzygy is hardly traceable. 



In an ordinary Antedon- f^yiygy there is no division of 

 the joint-face into two lateral portions as in the case just 

 described which is thus somewhat intermediate in its nat- 

 ure, presenting a distinct approach to a ligamentous ar- 



1) Phil, Trans, loc. cit. PI. XXXV 1. Hgs. 5 D, (3 1). 



2) Ibid. figs. 2B, 3A, 4B, 5A. 



3) liiiin. Trans, loc. cit. PI. 7. liiis. 2 1j, 8a, 5b, (j;i. 



4_) K. Vet. Akad. Furh. loc. cit. j). 230. lig. c. (Oonniarc also iig.s. (i, k, 1.) 



JN'otes from tlie Xjeyden Miuscum, "Vol. III. 



