100 BRITISH PALEOZOIC ASTEROZOA. 



Our knowledge of the Palaeozoic Asteroidea has greatly increased since Schondorf 

 published his researches, and it is now possible to draw comparisons between these 

 Devonian forms and the older Ordovician and Silurian genera. They resemble the 

 more advanced species of the Promopalaeasterinse, inasmuch as on the apical surface 

 the development of the intermarginal (interbrachial) area causes the first pairs of 

 supero-marginalia to be situate well within the disc-area (see Text-fig. 29, p. 34, 

 of the Introductory Section to this Monograph), whilst on the oral surface 

 several pairs of infero-marginalia are enclosed in the disc (Text-fig. 4, p. 12). 

 Schuchert (pp. 128 — 130) notes these resemblances, but points out that the 

 Xenasteridoe have accessory interbrachials (ventrolateralia ?) whilst the Promo- 

 palgeasterinee have no such plates. He also points out the considerable 

 resemblance between Devonaster (see above, p. 80) and Xenaster. According 

 to Schuchert (85, p. 130) : " If it were not for the accessory interbrachials, 

 Xenaster would be closely related to Promopalseaster, which also has a number of 

 inframarginals crowded into the interbrachial areas. It is the Promopalasasfer- 

 stock out of which Xenaster probably developed, while Devonaster apparently came 

 through Mesopalseaster.'" 



In my opinion, Schuchert is wrong here. The apparent similarity between the 

 interbrachial structures in the two series of forms is brought about by parallel 

 development. We have seen that the Promopalgeasterinae reach their maximum 

 development in the Upper Ordovician, and even at this early period begin to show 

 signs of degeneration. The specialised character of the mouth-parts of Pronio- 

 palxaster is not repeated in Xenaster, which has mouth-parts of the Recent Asteroid 

 type (Schondorf, ' Palgeontographica,' p. 89). Further, there is no known form 

 from the whole of the Silurian which would serve to link up the two series. The 

 Xenasteridge are undoubtedly related to Devonaster, and this relationship suggests 

 the derivation of both from a common Mesopaldeaster-stock some time during the 

 Silurian period. 



The family of Xenasteridae contains the following genera : Xenaster, Simono- 

 vitsch ; Agalmaster, Schondorf; Bhenasier, Schondorf; Eifelasfer, Schondorf; 

 Trimeraster, Schondorf. 



No member of the family has been found up to the present in either Britain or 

 America. 



End Speciks op Undetermined Lineages. 



Attention has already been called to the breaking down of the ossicles on the 

 apical surfaces in advanced forms such as those of the Promopalgsaster lineages. In 

 the following forms this irregular subdivision of the ossicles has apparently gone 

 still further, but unfortunately our knowledge is not sufficient for us to determine 

 the exact lineages to which the species belong. 



