224 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



Scutella striatula Conrad, 1857, U. S. Pacific R. R. Expl. 32d Par. and California, vol. 7, pi. IX, figs, la, lb; non 



Dendraster excentricus Dujardin and Hupe, 1862, Hist. nat. zoophytes echinodermes, p. 564. 



Scutella striatula Meek, 1864, Smithsonian Misc. Coll., vol. 7 (1S3), p. 2. 



Scutella striatula Gabb, 1869, California Geol. Survey, Paleontology, vol. 2, p. 110. 



Scutella (Echinaraehnius) excentricus A. Agassiz, 1872, Revision of the echini, Mus. Comp. Zoology Illus. Cat. No. 7, 



pp. 107, 524-526. PL Xllla, figs. 1 to 4. 

 Echinaraehnius excentricus Cooper, 1888, Cat. California fossils: State Mineralogist Seventh Rept., p. 271. 

 Echinaraehnius excentricus Gregory, 1891, Geol. Soc. America Bull., vol. 3, p. 107. 

 Echinaraehnius excentricus Merriam, 1899, California Acad. Sci. Proc, 3d ser., Geology, vol. 1, No. 5, p. 170, PL XXII, 



fig. 8. 

 Scutella (Echinaraehnius) excentricus Arnold, 1903, California Acad. Sci. Mem., vol. 3, p. 91. 

 (?) Echinaraehnius excentricus Eschscholtz var. Arnold, 1907, U. S. Geol. Survey Bull.. 322, PL XXIV, fig. 8. (This 



is probably a variant form of Dendraster gibbsii.) 

 Scutella striatula Schuchert, 1908, U. S. Nat. Mus. Bull. 53, pt. 1, p. 594. 



Echinaraehnius excentricus Pack, 1909, California Univ. Dept. Geology Bull., vol. 5, No. 13, pp. 231-282. 

 Dendraster excentricus Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 707. 



Determinative characters. — Test medium to large; subcircular to broadly subovate in margi- 

 nal outline, broader posteriorly than anteriorly; upper surface greatly depressed around the 

 margin, more or less tumid centrally; margin thin; under surface flat. Apex central or sub- 

 central. Apical sj^stem excentric posteriorly; from one-fourth to one-third the radius from 

 the center. Ambulaeral petals large; posterior pair shorter than the others, less divergent, 

 equally divergent, or slighthy more divergent than the anterior pair. Peristome small, circular, 

 central or subcentral; ambulaeral furrows fork near the peristome, the branches at first diverg- 

 ing then converging slightly, each giving off one important and many small branches on the way 

 to the margin. The stronger branches continue abactinally, but they are rarely discernible 

 on fossil specimens. Periproct very small, circular or subelhptical, inframarginal. 



Dimensions. — Specimen A: Length 83 millimeters: width S3 millimeters; height 12 milli- 

 meters. Specimen B: Length 97 millimeters; width 105 millimeters; height 19 millimeters. 



Description. — This well-known Recent form, abundant on the Pacific coast, is so fully 

 described in other works, that it is necessary to give here only the special points of interest in 

 regard to the fossil specimens. Most of these have already been given in the determinative 

 characters, but a few additional details will be presented. The size varies from less than 2 to 

 more than 4 inches in diameter and less than one-fourth to about 1 inch in height. In marginal 

 outline the smaller forms are usually subcircular while the large ones are usually broadly subovate 

 and broader than long. The apex is usually between the center and the apical system, but is 

 sometimes central or slightly excentric anteriorly. The area between the ends of the petals 

 and the margin is wider than in forms which resemble this. The under surface is flat or slightly 

 concave. 



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

 ambulacral areas; dorsal portions petaloid, the petals somewhat variable in details. The petals 

 are large, the anterior three longer than the posterior pair, the odd petal longer than the anterior 

 paired petals, though sometimes appearing of equal length or shorter by reason of the shortening 

 of the distance between the pairs of pores of the poriferous zones for a greater or less distance 

 from the end of the petal. The anterior paired petals are straight or nearly so and more widely 

 divergent or equally divergent or slightly less divergent than the posterior pair. The posterior 

 pair is usually less widely divergent and the poriferous zones of each petal are less widely diver- 

 gent than in similar forms. The interporiferous areas of all the petals are rather wide and some- 

 what tumid. 



The apical system 'is excentric posteriorly, being situated from one-fourth to one-third the 

 distance from the center to the posterior margin ; the madreporite is large and substellate with 

 four genital pores at the anterior points of the star, the two anterior nearer together than the 

 two posterior. There are also five small radial plates each perforated by a minute pore. 



The peristome is central or slightly excentric posteriorly. The ambulaeral furrows are simple 

 and straight for a very short distance from the peristome, then fork nearly symmetrically into 



