214 



BULLETIN 



UNITED STATES NATIONAL MUSEUM. 



Fig. 14.— a section through the arm of an aulurid, 

 "with alternating ambulacralia, after schon- 

 dorf. a, ambuxacralia of the dorsal and ven- 

 TRAL skeleton; ^1, VENTRAL EXTENSION OF THE 

 AMBULACRALIA ; A d, ADAMBULACRALLV = LATERAL 

 SmELDS OF OPHIURIDS; F, PODIA, ON LEFT THE CANAL 

 PASSES THROUGH THE PLATE, ON RIGHT THE PLATE IS 

 DISSECTED DOWN TO THE CANAL; TKr, INTERSKELETAL 

 RADIAL WATER- VESSEL. 



are covered with spines and tubercles.] Disk with concave, or con- 

 vex (?) mai'gins, with or without marginal ossicles. Wlien the latter 

 are present, they never extend along the rays, but are whoUy re- 

 stricted to the disk. The rays on both sides are margined by the 

 adambulacrals. Ventrally the rays have broad open ambulacral fur- 

 rows, bounded laterally by the 

 adambulacrals. A typical mad- 

 reporite [probably always] lies 

 in one of the ventral interradii. 

 "The group [subclass] is re- 

 stricted to the older Paleozoic." 

 Remarks. — This subclass of 

 Stelleroidea appears to be an in- 

 dependent development whose 

 structure partakes of that of 

 the Asteroidea and the Ophiu- 

 roidea, though more like the 

 latter. The Auluroidea agree 

 with the asterids in having open 

 ambulacral furrows, and a true madreporite, which is, however, ven- 

 trally situated, but otherwise the class is more like the ophiurids, smce 

 the body cavity does not extend into the rays as in true starfishes. 

 However, the water-vascular canal in the Auluroidea does not lie 

 outside of the ambulacrals as in the Asteroidea, but within these 

 ossicles along the mid-line of the ambulacrum m a canal that is cut 

 out of the sides of adjoining ambulacral columns. On the. other 

 hand, the Ophiu- 



roidea have the am- A6 A A t :> Ad ^ A Ad 



bulacralia coossified 

 and modified into 

 vertebral ossicles; 

 there are no open 

 ambulacral furrows, 

 and the entire rays 

 are covered by four 

 columns of shields or by an integument more or loss studded with 

 calcareous plates and granules. 



It appears that the Auluroidea all have dorsally four columns of 

 plates, of which the two medial ones are the most prominent. These 

 are undoubtedly the dorsal aspect of the thickened ambulacrals. 

 Outside of the disk they are usually convex and more or less orna- 

 mented with granules, but within the disk are less prominent aad take 

 on other expressions, commonly with concave surfaces. The plates 

 outside of these margin the rays and are the adambulacrals; while 

 in some forms they are prominent and tuberculatc, usually they are 

 made up of narrow vertical pieces that bear spines along the ventral 

 or only the distal ventral edge. 



Figs. 15 and 16.— Ventral and dorsal arm structihie of auluroidea, 



WITH the AMBXn-ACRA ALTERNATING. SCHEMATIC, AFTER SCHONDORF. 

 jl, AMBULACRALIA; ^', VENTRAL EXTENSIONS OF AMBULACRALIA.; AS, 

 AD AMBULACRALIA; FS, PODIAL CAVITIES. 



