130 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



Distribution. — Restricted to the Lower Devonic of Germany. 

 The species are : 



X. margaritatus Simonovitsch. 



X. dispar Schondorf . 



X. elegans Schondorf. 



X. {?) rhenanus (Miiller). 

 Remarks. — ^IVIiiller in his description of Asterias rlienana,^ the geno- 

 type of Archxasterias, notes that the adambulacral plates of adjoining 

 columns alternate and that the ambulacral columns ' ' have the appear- 

 ance also of alternating/' which if established would be very remark- 

 able, and would justify the creation of a distinct genus Archxasterias 

 for this asterid of the Rhine grauwacke. Schondorf shows that 

 these statements of Muller's are very faulty and further that the 

 genus Archseasterias has not been defined so as to be recognizable. 

 Under these circumstances it was best for Simonovitsch to disre- 

 gard Archxasterias and to estabhsh a new generic name for his species 

 margaritatus. There is no Paleozoic genus of starfishes better 

 described or illustrated than this one by Simonovitsch, and should 

 it ever prove that Asterias rTienana Miiller is identical with Xenaster 

 TMirgaritatus, a possibility, the latter should not be made to give 

 way to the former on the ground of priority. It is an injustice to 

 extend the law of priority indiscriminately to all work, and to reject 

 Archxasterias is no reflection on the work of Miiller, as his material 

 was very poor and his work of early date in paleontology. He pub- 

 lished at a time when it was very desirable to learn anything what- 

 ever about Paleozoic stai-fishes. 



As pointed out above, Xenaster has much of the structure of 

 Devonaster, yet it differs in the important character of having 

 numerous accessory interbrachial plates. This feature seemmgly 

 relates it with forms like Palasterina which have a great abundance 

 of accessory interbrachials, yet in no interbrachial area of this genus 

 are there axillary interbrachials or interbrachial marginals. In other 

 words, the Palasterina type of disk is made by the intercalation of 

 accessory plates between the inframarginals and adambulacrals, 

 whereas in Devonaster it is accomplished by the greater mdth of the 

 inframarginals and by the oral crowding of a plate which m primitive 

 forms is an axillary marginal. Devonaster although from younger 

 strata is more primitive than Xenaster because of its simpler inter- 

 brachial actinal structure. If it were not for the accessory inter- 

 brachials, Xenaster would be closely related to Promopalxaster which 

 also has a number of proximal inframarginals crowded mto the inter- 

 brachial areas. It is the Promopalxaster stock out of which Xenaster 

 probably developed, while Devonaster apparently came through 

 Mesopalxaster. 



» Verb, naturh. Ver. preuss. Rhelnl., etc., vol. 12, 1855, pp. 6, 8. 



