250 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



"Ray divided into about 40 segments or articulations by the 

 encircling, dorsal plates; these grow rapidly narrower near the body, 

 and are divided by a dorsal suture running along a raised median 

 ridge which extends about two-fifths of the length of the ray from 

 the body out, where it gradually disappears with the last divided 

 plate; the rest are entire. These plates are quadrangular, and have 

 their inner corners articulated into the socket formed in the dentate . 

 lateral extensions of the ambulacral series. They are ornamented 

 on each side by two or three closely arranged, subparallel, very fine 

 ridges, that commence near the dorsal line at the free margins and 

 extend diagonally backward across the plate toward the inner lateral 

 articulated corner. Ambulacral groove furnished with opposite, 

 irregularly quadrilateral, ambulacral plates, divided by a median 

 suture which is crossed alternately by one long and one short suture. 



"The superior plate of each pair has the greatest extension later- 

 ally, while in the inferior it is the direction of the median line, it 

 bemg about twice as long as the superior, which, however, is but 

 slightly broader. 



"Between the lateral margins of the dorsal plates and the dentate 

 elevations of the ambulacral plates are large subtrigonal open spaces. 



"The plates are punctured by very closely arranged minute 

 perforations." 



Formation and locality. — In the Rochester shale, at Lockport, New 

 York. The holotype is in the collection of Dr. Ringueberg. At 

 least one (No. 1110) and probably two other specimens (No. 465) 

 are in the Walker collection of the University of Toronto. They are 

 from the Rochester shale, at Grimsby, Ontario. 



Genus LAPWORTHURA Gregory. 



Text fig. 30. 



Lapworthura Gregory, Proc. Zool. Soc. London for 1896, 1897, p. 1037, fig. 5.— 

 ScHONDORF, Palseontographica, vol. 57, 1910, p. 58; Jahrb. nassauisch. Ver. 

 Naturk., Wiesbaden, vol. 63, 1910, p. 208.— Sollas and 

 SoLLAs, Philos. Trans. Roy. Soc. London, ser. B, vol. 

 202, 1912, p. 213.— Spencer, Mon. Brit. Pal. Asterozoa, 

 pt. 1 (Palfeontogr. Soc. for 1913), 1914, pp. 25, 32, 40, 49. 



Original diagnosis. — "Disk circular, well 

 marked. Arms very flexible, broad; at first 

 FIG. 30.-VENTEAI. STRUCTURE uuiform iu ^dth aud then tapering slowly. 

 OF ARM OF Lapworthura, Ambulacral ossicles with the distal and prox- 

 AFTER GREGORY. .^^^^ margius parallel ; mth lateral mngs curv- 



mg round the podial pores. Madreporite large," dorsal. 

 Genoliolotype. — Protaster miltoni Salter. 

 Schondorf remarks as follows (19106:58): 



The ossicles of Lapworthura have been described several times, 

 but never correctly. The ambulacrals arc undoubtedly opposite, 



