334 
ing genera . . . . shows that in general the older genera have the 
more tubular, simpler thecze, with the less protected apertural margins. It 
is, hence, apparent that the stolonal or earlier thecee of the rhabdosomes 
represent indeed the older types of thecal form.’ (48.) 
Other Classes.—The case of the larva of Antedon has already been 
referred to. As pointed out by Bather (1), the stem ossicles of the larval 
Antedon are of a complex and specialized type, and in a general way re- 
semble the stem ossicles of the Bourgueticrinidze of the Upper Cretaceous. 
It is held by Bather that the structures of the adult ancestors have been 
pushed back by acceleration to the larval stages of the existing Antedon. 
Recapitulation is also shown in the anal plate of Antedon. The anal 
plate appears between two of the radials and on the same level with them. 
Subsequently it is lifted out from between the radials, and the latter close 
beneath it. Still later the anal plate is resorbed entirely. That this is the 
recapitulation of an adult character and not of a larval character, as con- 
tended by Hurst, is shown by the fact that the oldest crinoids do not possess 
the anal plate at all. It appears from paleontological evidence that this 
plate first appeared above the level of the radials, that it gradually sank 
down between the two posterior radials, and that at a far later period 
(at about the close of the Paleozoic) it gradually passed upward again as 
it does in Antedon, and eventually disappeared. 
Jackson has shown that there is good evidence of recapitulation among 
the fossil echinoids (33). In most regions of the echinoid the develop- 
ment is obscured by the more or less extensive resorption, but the plates 
of the corona may show by their position and number, the course of devel- 
opment. Jackson holds that the introduction of columns of plates, both 
interambulacral, and ambulacral, in J/ceclonites, ete., indicates the stages of 
growth through which the individual has passed in its development. He 
shows that two columns of ambulacral plates “may be accepted as the 
usual characteristic of the whole class, which finds its representative in 
the majority of the adults, in nearly all young, and in the adult of the 
simplest and oldest known type, Bothriocidaris.” 
Interambulacral areas originate ventrally in a single plate. Only one 
genus is known, however, that has a single row of plates in the adult, 
namely Bothriocidaris. This is the simplest known and “perhaps the 
simplest conceivable echinoid.” 
