164 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



In the Quaternary deposits Scutella is replaced by an 

 Echinai-achnius which is a common form at the present 

 time. This species, E. excentrictis, PL XXII, fig. 8, is 

 probably nearly related to E. Gtbbst, PI. XXII, fig. 7, a 

 form cited from the middle or later Tertiary. E. Gibbsi 

 occurs near Buena Vista Lake, Kern County, in deposits 

 which have been referred to the Miocene, and is also 

 cited by Ashley from his transition beds between the Mer- 

 ced and the Miocene near Santa Cruz. This species much 

 resembles the excentrictis, but shows a greater degree of 

 eccentricity of the apical system than is found in that 

 species. The high degree of specialization in E. Gibbsi 

 is not easily explained when we consider that it is the 

 older form. As both forms are immigrants, and as their 

 life-periods do not appear to overlap in this region, they 

 are probably derived from a common stock, less specialized 

 than either form, which lived in some other Pacific region 

 during the middle Tertiary. 



The table on the page opposite illustrates the occurrence 

 and probable relationships of the known species of middle 

 Californian sea-urchins. Unbroken lines connecting spe- 

 cies indicate such structural and successional relationship 

 as suggests descent. Dotted lines indicate doubtful descent 

 or doubtful genetic relationship. 



III. Description of Species. 



Genus Schizaster A^assiz. 

 Schizaster Le Contei, sp. nov. 



Plate XXI, Figs, i and la. 

 Schizaster, sp., Journ. Geol., Vol. V, p. 773, Dec, 1897. 



Small forms averaging a little less than 20 mm. in length. The largest 

 specimen measured is about 23 mm. long. Test distinctly notched ante- 

 riorly by the groove of the anterior ambulacrum, truncated posteriorly; upper 

 surface much elevated, with a sharp ridge running from the apical system to 

 the posterior end, summit situated far back. Apical system eccentric, poste- 

 rior, anterior to the summit. 



