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University of California Publications in Geology [Vol. 12 



to the margin, somewhat resembling those found in the genus Asiro- 

 dapsis. The Scutella. gabbi group in the Upper Miocene has given 

 rise to several forms, and S. gabbi itself may be considered as a direct 

 descendant of ^S*. merriami, though differing only slightly. It shows 

 a somewhat more advanced stage in that the periproct tends to assume 

 a supramarginal position. Scutella gabbi var. tenuis Kew probably 



S. gabbi (notched form) 



S. gabbi Re'mond S. gabbi, var. tenuis Kew 



S. norrisi Pack 

 S. vaquerosensis (large form) 



S. tejonensis Kew 



S. fairbanksi, var. santanensis Kew 



S. andersoni Twitchell 



S. merriami (Anderson) 



S. blancoensis Kew 

 I 

 1 



S. vaquerosensis Kew 



S. fairbanksi Arnold 



S. ncwcombei Kew 



S. coosensis Kew 

 J 



I 



SCUTELLA 



Fig. 3. Phylogenetic tree for the Genus Scutella. 



is somewhat more specialized than 8. gabbi in that it is a larger form 

 and relatively much thinner. The other form of S. gabbi (notched 

 form) is later in time, but is hardly thought to represent even a 

 varietal difference inasmuch as it differs only in having a slight 

 notching of the margin in the posterior ambulacral areas. 8. blanco- 

 ensis Kew is a species similar to 8. gabbi, though it lived in an entirely 

 separate geographic province, viz., in the vicinity of Cape Blanco, 

 Oregon, during the Oligocene. It is somewhat less highly developed 

 than 8. gabbi, the periproct being entirely supramarginal and the 

 test thinner. The supramarginal position of the periproct in this 



