190 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



Genus DENDRASTER Agassiz emend. 



Dendraster L. Agassiz, 1847, Cat. raisorme echinodermes; Soc. Nat. Ann., vol. 7, p. 135. 



Echinarachnius A. Agassiz, 1S72 (subgenus) pars, Revision of the Echini, Mus. Coinp.. Zoology Illus. Cat. 7, pp. 107, 524. 

 Echinarachnius Duncan, 1891 (subgenus) pars, Revision genera and great groups Echinoidea, p. 158. 

 Echinarachnius Grabau and Shinier, 1910, North American index fossils, .p. 592. 



The genus Dendraster as used here is that of Agassiz with a slight amendment enlarging it. 

 It is here regarded as including the Scutella-like forms having a decidedly posteriorly excentric 

 apical system, very unequal petals, ambulacral furrows bifurcating near the peristome, and the 

 periproct infraniarginal, marginal, or supramargmal. The species D. excentricus is the type of 

 the genus. 



Some confusion exists in regard to the forms included in the present genus by reason of the 

 use of the generic term Echinarachnius by some authors as a subgenus of Scutella for the forms 

 having an excentric apical system. Duncan is partly responsible for this, having proposed it 

 in his revision of the Echinoidea. In Eastman's translation of Zittel's Paleontology the sug- 

 gestion of Duncan was carried out, and in Grabau and Shimer's "Index fossils of North 

 America" more positive action was taken by divorcing Echinarachnius from Scutella and still 

 ascribing it, and basing it essentially upon, the excentric apical system. 



The genus Echinarachnius was established by Leske in 1778, what is evidently his type 

 species being Echinus placenta Linne. Neither this species nor any of the allied forms he 

 incidentally mentioned had an excentric apical system. The species placenta is now recognized 

 (though with doubtful propriety) as the type of the genus Arachnoides Klein. (Seep. 124.) 

 Gray, in 1825, redrew and enlarged Echinarachnius Leske, retaining placenta as the type species 

 and adding Scutella parma Lamarck and Scutella lenticularis Lamarck, neither of which has an 

 excentric apical system. A. Agassiz, in 1872, reduced Echinarachnius to the rank of a sub- 

 genus under Scutella; distinguishing it chiefly by having "interior pillars more or less concentric 

 with the edge," and including both forms with central apical system, E. parma, and excentric 

 apical system, E. excentricus. In view of these facts it is difficult to see any grounds for the 

 use of the name Echinarachnius for forms with an excentric apical system. 



As a result of the discovery in California deposits of many specimens, including several 

 new species, having an excentric apical system, the writer has been led to differ with A. Agassiz 

 and regards the excentricity of the apical system as a fundamental and satisfactory basis of 

 separation and therefore regards the genus Dendraster L. Agassiz as well founded. Stef anani x 

 appears to have come to the same conclusion. 



Dendraster perrini (Weaver). 

 Plate LXXXVIII, figures 2, 3a-c. 



Scutella perrini Weaver, 1908, California Univ. Dept. Geology Bull., vol. 5, No. 17, p. 273, PI. XII, fig. 2. 

 Astrodapsis perrini Rathbun, 1908, U. S. Nat. Mus. Proc, vol. 35, p. 342. Listed on authority of R. Arnold and 



erroneously ascribed to Merriam. 

 Scutella perrini Arnold, 1909, U. S. Geol. Survey Geol. Atlas, Santa Cruz folio (No. 163), p. 6. 

 Scutella perrini Arnold, 1909, U. S. Geol. Survey Bull. 396, pp. 30, 34, 38, 162, PI. XXVIII, figs. 1, 2. 

 Echinodiscus (?) perrini Lambert, 1909, Rev. crit. paleozoologie, vol. 13, p. 122. 

 Scutella perrini Arnold and Anderson, 1910, U. S. Geol. Survey Bull. 398, p. 338, PL L, figs. 1, 2. 

 Merriamaster perrini Lambert, 1911, Rev. crit. paleozoologie, vol. 15, p. 64. 

 Merriamaster perrini Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 704. 



Determinative characters. — Test medium in size; usually circular rarely longitudinally oval 

 in marginal outline, broadest centrally; upper surface greatly depressed, slightly convex, rising 

 gradually from the unusually thick margin to the low apex which is slightly excentric anteriorly ; 

 under surface concave. Tubercles conspicuous, nearly equally so over the whole test; not 

 close together. Apical system excentric posteriorly, from about one-sixth to about one-fifth 

 the radius from the center. Ambulacral petals large, wide, broadly subelliptical, all of them 

 extending nearly to the margin, wide open at the ends; interporiferous areas very wide, some- 

 times faintly tumid, the posterior paired petals shorter than the others, though less so than in 



1 Stefanani, G., Soc. geol. italiana Boll., vol. 33, p. 704. 



