Geol.— Vol. I.] MERRIAM— TERTIARY SEA-URCHINS. 163 



seen in abundance within less than ten feet of each other 

 and may be found to overlap. A. tumidus does not show 

 much affinity to S. Gabbi, but is closely related to the 

 older form, Clypeaster Brezuerianits. From this species it 

 is distinguished mainly by the strong relief of the petals, 

 and it is probable that C. Brezuerianus is the ancestor of 

 A. tumidus. Since in the sequence of strata the beds 

 with S . Gabbi lie between those containing these two spe- 

 cies, and Clyfeastcr was absent during the intervening 

 period, the modification or evolution was probably brought 

 about in some other region, Astrodapsis afterwards com- 

 ing in to replace the disappearing Scutella. Closely re- 

 lated to A. tumidus is the larger and thinner A. Whit- 

 neyi, PI. XXI, fig. 4, which probably belongs to the later 

 portion of the San Pablo. Both species were confined to 

 this epoch, and with their extinction the more general- 

 ized Clypeastroids disappeared from this region. Along 

 the line of descent from Clypeaster Breiverianus through 

 Astrodapsis timiidus to A . Whitneyi the gradual increase in 

 size is quite noticeable. The maximum diameter of the 

 first species is 33 mm., of the second 45 mm., and of the 

 third 60-65 nim. 



During the Merced epoch, following the San Pablo, the 

 Scutellinae reappear, represented by Scutella interlineata, 

 PL XXII, fig. 6. This form shows considerable resemblance 

 to S . Gabbi and has in common with it the supramarginal 

 anus. S. interlineata differs from S. Gabbi in being 

 much larger, in the position of the apical system, which is 

 quite eccentric, and in the more pronounced supramarginal 

 character of the anus. These differences are in the direc- 

 tion of greater specialization, except the position of the 

 anus. This opening is travelling back from the lower sur- 

 face toward the apical shield. Taking into consideration the 

 points of relationship of these species, the fact that the dif- 

 ferences may be due to specialization of the older form, 

 and the order of occurrence of the two, it is probably safe 

 to assume that S . interlineata is a modified descendant of 

 S . Gabbi. 



