272 



University of California Publications. 



[Geology 



faint re-entrant angle opposite the anal opening. The petals are 

 broad and nearly closed at the extremity. The median inter- 

 poriferons zones are broad, while the outer rows of pores eon- 

 verge at the ends so as almost to enclose them. The tubercula- 

 tion is uniform on both the upper and lower surfaces and the 

 tubercules are of nearly the same size over the whole test. 



The actinal surface is strongly concave and the actinostome 

 deeply sunken. The ambulacral furrows are very deep and ex- 

 tend from the actinostome to the margin. 



Clypeaster bowersi differs considerably from the other mem- 

 bers of this genus on the Pacific coast. It is not far removed 

 from C. brewnigei Laube, which occurs in the Eocene in the 

 Lybian Desert of Africa. C. bowersi is nearly twice as large as 

 C. brewnigei and is ellipsoidal in outline with thick margins, 

 while the latter is pentagonal and has thin margins. The upper 

 surface of the former is only slightly arched, while the apical 

 system of the latter is conical and then gradually slopes to the 

 margin. They differ also in the character of the petals and in 

 the position of the anal pore. In C. bowersi the petals are broad 

 and the poriferous zones lie nearly parallel. The anus is situ- 

 ated on the actinal surface at a distance from the margin about 

 equal to its diameter. 



Clypeaster bowersi shows a close relationship to Echinanthus 

 rosace us Gray. It differs in that it is much flatter, the superior 

 surface of E. rosace us being more strongly convex. The actino- 

 stome of the former is sunken in a cavity which narrows grad- 

 ually and is of much greater extent than in E. rosace us. In 

 E. rosaceus the ambulacral areas are more or less swollen and 

 rise above the general level of the test, while in C. bowersi they 

 are nearly at the same level as the interambulacral areas. The 

 posterior ambulacral petals are the longest and the odd ambu- 

 lacral petal is the shortest in E. rosaceus. In C. bowersi the odd 

 ambulacral petal is the longest and the remaining four are of 

 equal size. 



Dimensions: Maximum width 100 mm.; maximum length 

 119 mm. ; maximum thickness 37 mm. 



Occurrence : Colorado Desert. Associated with a fauna pre- 

 sumed to be of Miocene age. 



