194 MESOZOIC AND CENOZOIC ECHINODERMATA OP THE UNITED STATES. 



suboval or subovate; length usually equal to or greater than width, though sometimes con- 

 siderably less, usually broadest back of the center but in front of the apical system. The upper 

 surface is somewhat irregularly convex; much depressed though less so posteriorly than ante- 

 riorly, rising directly from the margin to the low apex which is more or less posteriorly excen- 

 tric, sloping more steeply posteriorly than anteriorly. The margin is thin and is usually slightly 

 notched in the middle of the posterior end. The under surface is flat or slightly concave. 



The ambulacral areas are wide, widest at the margin, where they are wider than the inter- 

 ambulacral areas; dorsal portions petaloid. The petals are large, the total area covered by 

 them being greater relatively than in most related forms ; the anterior three longer than the pos- 

 terior two, the odd petal equal to or slightly longer than the anterior pair; all of them except 

 the odd petal approaching rather close to the margin. The anterior paired petals curve toward 

 each other, so as to be less divergent than the posterior pair, which are very short, widely 

 divergent, and have their anterior side the longer and their poriferous zones widely divergent. 

 The poriferous zones are of moderate width, those of the odd petal usually being narrower than 

 the rest; pores oval and conjugated. The interporiferous areas are wide, much wider than the 

 poriferous zones. 



The interambulacral areas are very narrow between the petals, wide at the margin. The 

 surface of the test is covered with small uniform perforate tubercles which are slightly larger 

 on the under surface. 



The apical system is very excentric posteriorly, being situated from one-third to more 

 than one-half the distance from the center to the posterior margin. The specimens which 

 are larger and more elongate longitudinally are those with the most excentric apical system. 

 The madreporite is large, subpentagonal, with four genital pores as the anterior points of the 

 pentagon, the two anterior nearer together than the two posterior. There are also five very 

 small radial plates each perforated by a minute pore. 



The peristome is small, excentric posteriorly, though decidedly less so than the apical system. 

 The ambulacral furrows are simple and straight for a short distance from the peristome, 

 theti fork nearly symmetrically into two strong furrows which diverge for a short distance, 

 then converge slightly and continue nearly to the margin. In young specimens they diverge 

 continuously to the margin. There do not appear to be many branches given off, though the 

 specimens are rarely sufficiently well preserved to show this feature. 



The periproct is very small, circular or subcircular, inframarginal, nearly marginal, usually 

 1 or 2 millimeters from the inner edge of the slight notch in the posterior border. 



Related forms. — Dendraster gibbsii is most closely related to D. excentricus, but differs 

 from the latter in having its apex less central, its apical system more excentric, its posterior petals 

 less widely divergent and with less widely divergent poriferous zones, and its periproct usually 

 somewhat nearer to the border. Some of the younger specimens of D. gibbsii also resemble 

 D. coalingaensis , but can be separated from it by its more excentric apical system, its less sym- 

 metrically elliptical ambulacral petals, and its more divergent posterior petals. No foreign 

 forms appear to be closely related to D. gibbsii. The Patagonian species Scutella patagonensis 

 Desor ] resembles D. gibbsii in occurrence in two quite different forms, one longitudinally 

 elongate and one transversely elongate, which yet evidently belong to the same species, for 

 they grade into one another and they occur together. 



Localities. — Kern Lake, Buena Vista County (Remond) ; near Buena Vista Lake, Kern 

 County (Gabb, Merriam); near Santa Cruz (Ashley); 8 miles north of Coahnga (PL LXXXIX, 

 figs, la, lb); ridge south of Garcas County, Coahnga district (PL LXXXIX, figs. 2, 3) ; 10 to 12 

 miles southeast of northwest end of Kettleman Hills, Coahnga district (PL LXXXIX, fig. 4) ; 

 numerous other localities in Coahnga district (including U. S. Geol. Survey localities Nos. 3S49, 

 4751, 4753, 4755, 4757, 4767, 4768, and 4806); San Jacinto; Capitola (Pack); San Gregorio 

 (Pack); Graciosa Ridge near Orcutt, Santa Maria district, California. 



1 See Reports of the Princeton University expeditions to Patagonia, 1896-1899, vol. 4, pt. 2, pp. 55-60; PI. XI, figs. 4a, b, c, d, e. 



