508 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 56. 



regularly spaced concentric fine threads with wider interspaces and 

 covered by a microscopic decussation which is found over the whole 

 surface of the valves ; anterior slits 11 or 12 ; posterior valve with a 

 low, nearly smooth mucro, slightly in advance of the center and 

 uniform concentric sculpture, as in the anterior valve; there is no 

 defined central area ; posterior slits 10 or 11 ; intermediate valves 

 with one slit at each end, the lateral areas slightly convex but not 

 sharply delimited, with the jugal and pleural tracts not separated or 

 limited, the sculpture the same as on the other valves, but the con- 

 centric threads slightly wavy; interior pink, the jugal sinus very 

 shallow, wide, indicated by a small notch at each end, straight edged, 

 smooth; the sutural laminae narrow, elongated; length of shell, 11; 

 width, 6.5 ; height, 4 mm. 



Habitat. — Near Cape San Lucas, Lower California, in 10 fathoms. 

 Cat. No. 218814, U.S.N.M. 



This is of the same general type as /. Ihnaciformis, but has a sculp- 

 ture totally diiferent from that of that species or /. fallax. 



ISCHNOCHITON COOPERI ACUTIOR Carpenter, MS. 



On comparing specimens named by Carpenter in the collection of 

 the United States National Museum, the only differences I could per- 

 ceive were that the specimens of the variety acutior were lighter and 

 brighter in color, more emphatic in sculpture, and apparently 

 younger shells. In a group where color is often without systematic 

 value, these differences seem hardly worth}'^ of a name, analogous 

 mutations being most common among Chitons. 



ISCHNOCHITON (RADSIELLA) AETHALOTUS, new species. 



Chiton small, flattish, wide, both valves and girdle of an olivaceous 

 tint, the latter densely covered with small oval imbricating scales: 

 anterior valve with about 18 irregularly spaced slits, less than semi- 

 circular, sculptured with fine, close, minutely divaricating incised 

 lines, among which are sparsely scattered relatively large brownish 

 (ocular ?) pustules; this sculpture is extended over the whole sur- 

 face of all the valves, the pustules showing no regularity, except a 

 somewhat radial arrangement on the posterior valve; there is no 

 segregated jugal tract and the lateral areas are feebly indicated ; pos- 

 terior valve with 11 slits, the mucro very obscure, subcentral, the in- 

 sertion plates smooth, the sutural laminae small, the jugal sinus shal- 

 low, straight-edged, smooth inside; intermediate valves with 2 

 slits, the first valve axially wider than the second, all with the jugal 

 sinus not defined; the sutural laminae rather wide, the valves not 

 mucronate ; interior color greenish ; gill rows ambient. Length, 12 ; 

 width, 6 ; height, 2 mm. 



Habitat.— G\\\i of California, W. J. Fisher. Cat. No. 58806c. 

 U.S.N.M. 



