8 BULLETIN 122, UNITED STATES NATIONAL MUSEUM. 



the later part they are scarcely as wide as the ridges. The spaces be- 

 tween the ridges are marked by fine striations which coincide with 

 the ridges. These markings are best shown on the younger portions 

 of the shell. The ridges have their free margin minutely denticu- 

 lated. In cross section they would appear triangular, with the 

 dorsal side about one-third as long as the ventral. The anterior 

 median part is marked by rather broad, strongly denticulated ridges 

 which join those of the anterior part in a little more than a right 

 angle. The junction of the anterior dental ridges and those of the 

 posterior median part form a well-impressed line which extends from 

 the ventral margin to the umbones. The spaces separating the dental 

 ridges on the anterior median portion are mere impressed lines. 

 The anterior half of the median portion is convex, while the posterior 

 half is depressed, forming a groove. Both parts are crossed by 

 rough, cordlike incremental lines, which follow a more or less sinu- 

 ous course. The posterior portion of the median part is about as 

 wide as the anterior and the median portion taken together, and is 

 crossed by oblique, somewhat wrinkled lines of growth. The pos- 

 terior part forms a strong auricle, which is thin, decidedly expanded 

 and slightly reflected at the margin, and emarginate at the umbonal 

 terminus. The interior is bluish white. The umbone marks a strong 

 knob from the underside of which the broad blade projects for a little 

 more than half the length of the shell. The ventral portion of the 

 median part bears the usual strong knob. The anterior and the me- 

 dian part show a strong tumid area at their junction, while the me- 

 dian portion of the middle part is concaved. The posterior part pro- 

 jects over the posterior median part as a strong shelf. This part in 

 cross section would show a decidedly sinuous outline. The emargi- 

 nation of the dorsal portion of the auricle shows best when seen from 

 within. The pallets form long, plumelike elements composed of a se- 

 ries of cup-in-cup structures, which project at the lateral margin in 

 the form of strong awns. Each cup is covered by a thin periostracum 

 which is faintly fimbriated at the distal end, and marked by longitu- 

 dinal striations. The stalk is about one-fourth the length of the 

 entire pallet. 



The specimen figured. Cat. No. 102762, U.S.N.M., comes from San 

 Francisco Bay. It measures: Height, 11 mm.; length, 12.5 mm.; 

 diameter, 11 mm. The pallet measures: Entire length, 32.5 mm.; 

 length of stalk, 7.5 mm.; diameter, 3.7 mm. The specimen is a 

 medium-sized individual. The animal, however, is probably the 

 longest of all the American shipworms and is said to attain a length 

 of 4 feet. 



We have seen this species from various stations from Unalaska to 

 San Francisco Bay. 



