SEA-URCHIXS. ^^' 



gerontic degeneration of armour (see Oswald, 61). Latueruc is 

 also allied to Cmihnlectypns, but, unlike most of the Holectypinse, 

 is almost globular in shape. The position of Discholectj/pus is 

 more difficult to determine. In every obvious feature it is a true 

 Holectypine, but it shows an arabulacral structure closely resem- 

 bling that of Conidas. The absolute contrasts of tuberculation, 

 periproct-position, apical structure, and general form which 

 a.j)pear wlien Discholectypus and Conidus are compared, preclude 

 any possibility of a genetic connection between the genera. 

 jyischolectyjms would seem, therefore, to be a branch of the 

 //olpctijpiis-Mne, which developed complex ambulacral plating by 

 a process of acceleration. This parallelism of development 

 (heterogenetic homfeomorphy) of a feature in two distinct genera 

 is rendered particularly interesting by the correspondence in tmie 

 at which the specialization took place. ^ 



There can he no doubt as to the close relationship wlucn exists 

 between Discoidea and HoUctypus. On stratigraphical evidence, 

 and also because of the variable nature of the apical system (in 

 the matter of the perforation of the posterior genital plate), I 

 have considered the Discoidiinse as descendants of Uolectypus 

 sens, str., whose appearance coincided in time with the modi- 

 fication of the parent stock into Ccenholectypus. 



5. S u m m a r y of Internal E v o 1 u t i o n. 



The Holectypoida originated from a Diademoid ancestor in the 

 Triassic or early Liassic periods, and subsequently developed along 

 two definite lines. In one line (Pygasteridjy and Conulidie) the 

 apical system never fully regained, and finally lost, the posterior 

 genital plate, while the whole system tended to become elongated ; 

 the tuberculation gradually became uniformly distributed over 

 the interambulacra, and irregular in its arrangement ; the shape 

 of the test showed various departures from radial symmetry ; and 

 the jaw-structures dwindled and ultimately almost disappeared in 

 adults. In the other line (the Discoidiida^), the fifth genital plate 

 was early redeveloped, and later regained its function, while the 

 system as a whole became circular in shape ; the tuberculation 

 retained its regularity of arrangement, but became insignificant 

 adapically and'coarse adorally ; the shape of the test eventually 

 regained a radial symmetry ; and the jaws, though modified, 

 showed little or no decrease in power. 



VI. The External Affinities of the Order. 



The primitive character of the early Holectypoida (in an 

 Irregular sense) is so pronounced that it would naturally be 

 expected that the group existed for some time before any of the 

 more elaborate forms were evolved, and that these appeared at 

 subsequent intervals as offshoots from the Ilolectypoid stock. 

 Such, however, was not quite the case. The Holectypoids are 



