APR 6 1898 



THE TERTIARY SEA-URCHINS OF MIDDLE 



CALIFORNIA. 



BY JOHN C. MERRIAM. 



CONTENTS. 



Page. 

 Plates XXI-XXII. 



I. Introduction i6i 



II. History and Relationships of Species 162 



III. Description of Species 165 



Explanation of Plates 172 



I. Introduction. 



In the course of an investigation of the invertebrate 

 faunas of Middle California, the writer has found the exist- 

 ing figures and descriptions of the sea-urchins either incom- 

 plete or inaccurate to such an extent as to make desirable 

 their reintroduction to those interested in the palaeontology 

 of this region. The writer by no means desires to cast 

 reflections on the work of those who first brought the spe- 

 cies to light, realizing that with abundant material at hand 

 it is much easier to revise than is the work of oriorinal 

 description. 



Both geologically and biologicall}' the sea-urchins are 

 among the most deserving of attention of the Californian 

 invertebrates. The comparatively short vertical range of 

 the species, together with their usually good preservation, 

 makes them admirable horizon determiners, while the inti- 

 mate relationship of some of the forms to each other indi- 

 cates considerable possibilities in the study of the history 

 and evolution of the clypeastroid branch of the class on 

 this coast, when the faunas of adjoining regions are better 

 known. 



[161I March 3, 1899. 



