Vol. XI] BERRY— FOSSIL CHITONS 



433 



Eaves on all valves badly worn, but apparently rather spongy. 



Measurements: Long, of head valve 6.+ mm., diameter 

 13+ mm. ; long, of median valve 6.0 mm, diameter 20.0 mm. ; 

 long, of tail valve [608] 10.4, [939] 11. mm., diameter [608] 

 18.+, [939] 19.4+ mm. 



Remarks: The specimens upon which the description of the 

 present species is based, though none of them entire, have 

 their various fragments for the most part in a fairly good state 

 of preservation. The characters in which they are peculiar are 

 not very striking, and it is therefore quite difficult at the present 

 time to arrive at any precise determination of their relation- 

 ships. The numerous slits, as well as the shorter teeth, pre- 

 clude reference to To nice! la, and recall in some degree the de- 

 scription of the two species of Spongioradsia (aleutica and mul- 

 tidentata), but the very spongy shell substance described for 

 the latter group, and the fact that the teeth are even more poorly 

 developed than in the fossil forms, militates against any at- 

 tempt to join them. In other respects likewise, Oligochiton and 

 Spongioradsia do not seem to have much in common. Cyauo- 

 plax and Lepidochitona have a differently formed tail valve, 

 longer teeth, and a wider separation of the much better de- 

 veloped sutural laminae. Nevertheless it is clear that the asso- 

 ciation of the Oligocene chiton should be more or less inti- 

 mately with the genera named in the family Callichitonidcc. 

 The comparatively weak development of the sutural laminae 

 and insertion plates perhaps indicates that Oligochiton is a 

 primitive form, perhaps ancestral to some of the modern mem- 

 bers of the family. 



It is of particular interest as the oldest chiton thus far re- 

 corded from the West American formations. 



Genus Tonicella Carpenter, 1873 • 



3. Tonicella lineata (Wood, 1815) 



(Plate II, figs. 1-5.) 



1815. Chiton lincatus Wood, Gen. Conch., p. 15, pi. 2, f. 4-5. 

 1847. Chiton lineatus Middendorff, Mai. Ross., I, p 109 pi 



12, f. 8-9. 

 1847. Chiton lineatus Sowerby, Conch. 111., f. 77. 

 1847. Chiton lineatus Reeve, Conch. Icon., v. 4, Chiton, sp. 



33, pi. 7, f. 33; detail pi., f. 33. 



