fig. 52; PI. YI., fig. 59, 2^mh. 1 mid iwih. 2). The laffer 

 joint is axillary, and supports, in each radius, two long 

 series of secundibrachials (PI. I., fig. 3, sec), which form 

 the skeletons of the five pairs of arms, and to these the 

 much shorter series of joints which support the pinnules 

 are articulated. 



Several specimens of Antedon with only eight arms 

 have been observed by the writer, and one with twelve was 

 described by Dendy (4). The eight-armed condition results 

 from the absence of one of the radii, while the two extra 

 arms of the twelve-armed specimen resulted from the 

 bifurcation of two of the normal ones. In each of these 

 the second secundibrachial became an axillary, similar to 

 the second primibrachials, and of which each facet bore 

 the long series of, in this case tertibrachial, joints forming 

 the arms. In many species of Antedon one or both facets 

 of the primibrachial axillary bear a series of two or three 

 secundibrachial joints, the second or third of which is an 

 axillary, and may in turn bear two or three tertibrachial 

 joints, the second or third of which is also an axillary. 

 In this way the large number of arms borne by 

 many tropical species arise. All the skeletal plates are 

 composed of more or less densely reticulate calcareous 

 matter, to which the name " stereom " has been given. 

 This reticulate structure may be seen in its simplest form 

 in tlie skeletal plates of the larva (PI. VI., figs. 66, 67, and 

 69), in which the reticulations are all in one plane, but in 

 the much more massive plates of the adult they form a 

 sort of sponge work, the meshes of which vary in shape 

 and size even in the same plate. 



The Centro-dorsal Plate. — The form of this plate 

 fPl. I., figs. 3 and 19) resembles that of a shallow bowl. 

 It is of considerable thickness; and its outer margin, as 

 well as that which bounds and slightly overhangs the 



