214 



BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



Ad. 



FIG. 14. A SECTION THBOUGH THE ARM OF AN AULUBID, 

 WITH ALTERNATING AMBULACEALIA, AFTEE SCHON- 

 DOEF. A, AMBULACEALIA OF THE DOESAL AND VEN- 

 TRAL SKELETON; A 1 , VENTEAL EXTENSION OF THE 

 AMBULACEALIA; Ad, AD AMBULACEALIA = LATERAL 



SHIELDS OF OPHIUEIDS; F, PODIA, ON LEFT THE CANAL 

 PASSES THBOUGH THE PLATE, ON EIGHT THE PLATE IS 

 DISSECTED DOWN TO THE CANAL; Wr, INTEESKELETAL 

 EADIAL WATEE- VESSEL. 



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

 vex ( ?) margins, with or without marginal ossicles. When the latter 

 are present, they never extend along the rays, but are wholly 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, since 

 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 in a canal that is cut 

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

 hand, the Ophiu- 



roidea have the am- AA A A i* A4 A 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 less 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 and 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 tuberculate, 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. VENTEAL AND DOESAL AEM STBUCTUEE OF AULUBOIDEA, 

 WITH THE AMBULACEA ALTEENATING. SCHEMATIC, AFTEE SCHSNDOEF. 

 A, AMBULACEALIA; A 1 , VENTEAL EXTENSIONS OF AMBULACEALIA; A 3, 

 AD AMBULACEALIA; FS, PODIAL CAVITIES. 



