PLAXIPHORA. 329 



margins sloping backwards into a point, crenulated on the sides ; 

 anterior valve with 9 radiating ridges crossed by fine concentric 

 zig-zag striae ; lateral areas with two, on each side, radiating ridges 

 crossed by fine zig-zag striae ; posterior and median areas with very 

 fine oblique striae diverging from the dorsal line outward and forward 

 crossed by others diverging outward and backward forming an 

 engine-turned pattern. Length 88 inch. ; breadth 31 inch. 



Color, mantle. white ; valves greyish-black with a white stripe on 

 each side of the dorsal line; interior greenish-blue. 



A single specimen is in the Colonial Museum, locality not stated. 

 (Hutton.) 



The following species probably belongs here, but I have seen no 

 specimen, and it has not been figured. 



CHITON (CH.ETOPLEURA) PLUMOSUS Gould. 



Shell brown-ashy, valves concave scarcely beaked ; anterior valve 

 ornamented wrth plumose radiating lirae; posterior valve small, the 

 apex marginal and having a submarginal rib on each side. Central 

 area large/ornamented with divaricating striae, and lateral plumosely 

 co-united striae. Lateral areas small, bounded by a sutural rib, striae 

 denticulated, interspaces clathrate. Girdle broad, leathery, furnished 

 with (bristly?) tubules. 



Length 1'5, breadth '75 inch. (Old) 



Habitat unknown. 



Allied to C. ccelatus Rv., which is said to be highly ornamented 

 with green and pink. C. collei and C. muscosus G. belong to the 

 same group. (Old., in Proc. Bost. Soc. Nat. Hist, vii, p. 165.) 



Section Guildingia Cpr., 1893, n. sect. 



Guildingia CPR., MS., and in Dall, Proc. U. S. Nat. Mus. 1881, 

 pp. 284, 288 (type G. obtecta Cpr. MS.) 



Shell and girdle like Plaxiphora, except that the valves are partly 

 immersed by the encroachment of the girdle. 



This section, like Fanneitia in the genus Tonicia, rests upon a 

 character of very little taxonomic value ; but as it is useful in break- 

 ing up a large genus for convenience in identifying, it has been 

 retained. 



