296 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 8 



Clypeaster rotundus (A. Agassiz) 

 Plate 47, Fig. 28 



Stolonoclypus rotundus A. Agassiz, 1863, p. 25. 

 Clypeaster rotundus A. Agassiz, 1872a, p, 100. 



H. L. Clark, 1914, p. 38, pis. 132, 133. 



Although adult specimens of this Clypeaster are easily distinguished 

 from the preceding, the young, before reaching a length of 25 mm are very 

 difficult to determine. They are nearly or quite circular, or a little longer 

 than wide, and very flat. The lower surface, at first nearly white becomes 

 yellowish, and as a rule is soon yellowish-brown, as in europacificus. The 

 subsequent color changes are similar in the two species but the body form 

 is usually characteristic, for europacificus has the interradial margins more 

 or less evidently concave or at least straight, resulting in a pentagonal 

 form with the length only a little, if at all, greater than the breadth. In 

 rotundus the interradial margins are more or less convex, and the outline 

 may be nearly circular, but as a rule the length is somewhat greater than 

 the width. The largest specimen in the present collection is 175 mm by 

 160 ; the height is 27 mm, but a second specimen with the same length and 

 breadth is only 20 mm high. The most nearly circular adult at hand is 146 

 mm long by 137 mm wide, but a specimen 37 mm long is 36 mm wide. 



Distribution. — There are 96 specimens in the Velero collection from 

 35 stations, well scattered along the coast from Santa Elena Bay, Ecuador 

 to Georgia Island at the upper end of the Gulf of California. There are 

 no records from the western coast of Lower California nor from anywhere 

 above the Mexican boundary. There are 9 specimens from Clarion Island 

 but none from Socorro, From Cocos Island there are two small specimens, 

 dredged in relatively deep water, 30-50 fms. There are 20 specimens from 

 9 stations in the Galapagos Islands, but chiefly from Tagus Cove, Albe- 

 marle Island. Some were taken in shallow water along shore but they 

 were mostly dredged in 8-50 fms. 



Type.— M.C.Z. no. 2188. 



Type locality. — Acapulco, Mexico. 



Depth. — Shore to 50 fms. 



Specimens examined. — 96 specimens from 35 stations. 



