514 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 55. 



the mucro, the anterior sinus also narrow and acute, the sutural lam- 

 inae broad; jugal and pleural areas of the intermediate valves not 

 separated, the sculpture of oblique reticulation with emphatically 

 punctate interstices; lateral areas similarly sculptured, bounded on 

 each side b}^ a slender rib, internally whitish with a narrow notch- 

 like sinus. Length, 12 ; width, 6 ; height, 4 mm. 



Habitat. — Granite Cove, Port Althorp, Alaska, in 16 fathoms, 

 gravel, collected by W. H. Dall. Cat. No. 208703, U.S.N.M. 



None other of the northern species has such a deep angular pos- 

 terior sinus. 



MOPALIA CELETOIDES, new species. 



Chiton with sculpture almost exactly like No. 218770, except that 

 the latter has more globular nodules on the lateral ribs and the pleural 

 tracts are less coarsely and evenly reticulated. In the present species 

 the anterior valve has 10 radial ribs with wider channeled interspaces 

 minutely reticulated ; the posterior valve, however, entirely different, 

 having a posterior mucro with a small rounded sinus below it, and no 

 ribs, the surface being taken up by the central area which has very 

 straight axial threads minutely annulated, with much finer and closer 

 transverse threads visible in the interspaces; gillrows extending for- 

 ward as far as the fourth valve ; posterior valve with 2 slits, anterior 

 with 8, intermediate with one at each end ; intermediate valves with 

 the lateral areas bounded by two strong ribs, the posterior wider, 

 denticulating the posterior margin; the interspaces minutely re- 

 ticulated; jugum minute with a small smooth mucro, the back slightly 

 keeled in front of it, the pleural tracts with about 11 strong axial 

 threads with wider interspaces crossed by regular equal minute 

 threads. Length, 12 ; width, 5.5 ; height, 2.5 mm. 



Hahitat.— Forrester Island, Alaska ; G. Willett. Cat. No. 218771, 

 U.S.N.M. 



This at first glance looks very unlike a Mopalia. Compare Cal- 

 listochiton celetus. 



Semimopalia, new subgenus. 

 Anterior valves slit, posterior valves with entire insertion plates. 



MOPALIA (SEMIMOPALIA) GRISEA, new species. 



Chiton of moderate size with pale olive-gray valves streaked with 

 a darker shade and with a series of pale rays on the dorsal line; 

 girdle leathery with sparse, rather long, often duplex hairs ; anterior 

 valve with 7 or 8 slits, posterior valve with no slits, anterior four in- 

 termediate valves with one slit and the fifth and sixth without slits, 

 the insertion plates smooth; anterior valve with 11 or 12 radii, the 

 surface minutely decussate; posterior valve much smaller with a 

 feeble posterior sinus, the low mucro at the posterior fourth, the 



