126 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



Generic characters. — Disk small, with apparently small interbrachial 

 arcs. Rays five, of moderate length and tapering at first slowly 

 and then rapidly distally. 



Abactinal area unknown. 



Inframarginal plates small, the columns not attaining the axils, 

 but resting upon the second enlarged axillary adambulacral plate. 



Adambulacral plates distally like the inframarginals, increasing 

 m size slowly proximally until near the axillary region, where four 

 plates suddenly attain great width, terminating in the axils of the 

 rays and here suppressing the inframarginal plates. Eight inter- 

 brachial adambulacrals of varying form are situated orally to the two 

 large axillary adambulacral plates, and at first give one the impression 

 of slender interbrachial areas of the same nature as in Promopalseaster. 

 These plates, however, are not interbrachial marginals, as in other 

 genera, and derived by the oral crowding of proximal inframarginals. 

 They are distinctly adambulacral ossicles, forming slight interbrachial 

 areas, and, with the axillary adambulacrals, form small arcs. For the 

 shape and position of these plates see figure 1, plate 20. 



Ambulacral plates alternating, slightly carinated, and very short 

 but wide. In the distal region they are as long as the adambulacral 

 plates, but proximally decrease in length where there are nearly two 

 of them to one adambulacral ossicle. . Podial openings between every 

 other plate through the sutures of adjoining pieces at their outer 

 edge and immediately beneath the inner edge of the adambulacral 

 ossicles. The proximal plates of each column are modified into oral 

 armature pieces that are slightly tumid, as long as wide, and sub- 

 triangular in outline. Between the proximal ambulacrals and orad 

 to the proximal interbrachial adambulacrals are situated additional 

 single, very small, quadrangular oral armature plates. 



Genoholotype and only species. — Palseaster miamiensis Miller. 



Distribution. — Ordovicic, Richmond formation, near Waynesville, 

 Ohio. 



Remarlcs. — The suppression of the inframarginal plates in the axils 

 of the rays by two of the adambulacral plates of each column and 

 the oral continuation beneath the latter of four pairs of adambulacral 

 plates holding an interbrachial position distinguish this genus 

 from Promopalseaster, to which it otherwise seems very closely related. 

 This character also distinguishes it from all other associated genera. 



The interbrachial structure of Anorthaster appears abnormal. In 

 Mesopalxaster and Promopalxaster the proximal inframarginal plates 

 are crowded orally and form interbrachial areas; in Anorthaster their 

 formation is effected in the opposite way, not by the inframarginals, 

 but by the adambulacrals. This character is seen in a single speci- 

 men, but it is so regularly developed and well preserved as to indicate 

 a normal and not a pathologic or abnormal condition. 



