190 CHITON-RADSIA. 



Carpenter's opinion of the small importance of the character upon 

 which this group is founded, is fully supported by the material 

 studied by myself. He writes as follows : " The bi- or tri-slitting 

 of the laminae appears to me a character of very secondary impor- 

 tance, as being merely numerical, not structural, and as being very 

 variable even in the same specimen. In one specimen of Dr. Gray's 

 type which I received from Mr. Cuming, only one valve had two slits 

 on each side; most of the other valves had one only ; sometimes a 

 second slit was seen on one side. For such a specimen it would have 

 been as useless to establish a genus as for Ch. virgulatus, or for C. 

 boivenii in which Dr. Gray found the same abnormality (vide P. Z. 

 S. 1847, p. 127). Another specimen of C. barnesii had the valves 

 either with one slit or with two slits on each side, or with one on one 

 side and two or three on the other. Under these circumstances I 

 cannot regard the number of slits as a generic character." 



Compare alse the notes under C. virgulatus. 



Key to species of Radsia. 



a. Surface smooth, goodalli. 



aa. Surface sculptured with riblets. 



b. Ribs strong and coarse ; color black-olive, uniform, 



sulcatus. 



bb. Riblets fine; color variegated. 



c. Ribets of central areas nearly obsolete ; of lateral areas 



granose, barnesii. 



cc. Riblets of central areas fine, distinct ; shell olive mottled 



with dark, virgulatus, 



C. BARNESII Gray. PL 29, figs. 10, 11, 12. 



Shell oval, depressed, scarcely carinated. Black-brown, yellowish 

 toward the ridge where each valve generally has a dark triangular 

 spot, and having a buff-white longitudinal stripe on each side ; 

 girdle green. 



Lateral areas raised, sculptured with about 6 irregular, uneven, 

 rather nodulous radiating ribs. Central areas sculptured with fine 

 longitudinal riblets, subobsolete toward the dorsal ridge, and some- 

 what latticed by growth-lines. End valves having radiating 

 nodulous ribs; umbo of the posterior valve very near the front mart/in. 



Interior whitish ; sinus finely and rather unevenly toothed. 

 Anterior valve having 15-16, central valves 2, (occasionally 1 on 



