70 Proceedings of the Biological Society oj Washington. 



extreme anterior edge, followed by the main anterior portion which bears 

 dental ridges and is about two and a half times as broad as the part just 

 mentioned. The dental ridges are of quite regular width and spacing; 

 they curve upward at their anterior limit, then extend in an even curve 

 across the shell to meet the anterior end of the dental ridges of the 

 median portion. In the type, 125 of these dental ridges are present. 

 These ridges are about as wide as the grooves that separate them. They 

 are triangular with the dorsal slope a little more abrupt than the vertical, 

 the edge being finely serrated. The depressions between the denticles 

 pass down on both sides of the dental ridge as fine incised lines. There 

 are about six denticles present on the ridge in a distance equal to that 

 separating ridge from ridge. The dental ridges of the anterior and me- 

 dian part meet almost at right angles. The denticles on the median 

 part are slightly stronger than those on the anterior ridges. The dental 

 ridges of the median portion terminate rather abruptly without change 

 of curve at the junction of the anterior median with the posterior median 

 portion. The posterior median portion is marked by coarse transverse 

 wrinkles on its anterior third, from which lesser lines of growth take a 

 sudden upward curve and extend partly over the rest of the shell. The 

 posterior median portion is suddenly deflected inward, giving this por- 

 tion of the shell a truncated appearance at the posterior margin. This 

 part is finely granulose above the median portion of the posterior auricular 

 part, while ventrally it is faintly longitudinally grooved. The posterior 

 portion of the shell is produced into a short auricle, which is marked by 

 the continuation of the feeble grooves extending over it from the pos- 

 terior portion of the median part. 

 Interior : 



The umbonal knob has a strong, narrow, blade-like tooth extending 

 obliquely into the cavity. A strong oblique shelf for muscular attach- 

 ment extends from the umbonal tooth to the ventral termination of the 

 posterior auricle, overarching the umbonal cavity. The blade for mus- 

 cular attachment is thin, short and decidedly flattened. It is inserted 

 under the umbonal tooth, extending obliquely backward, the tip point- 

 ing to a position midway on the ventral edge of the posterior part. In 

 Teredo s. s. the blade usually has its flat surface parallel to the ventral 

 curvature of the interior of the shell, while here the revers is almost 

 true, for the narrow edge almost parallels the interior. The extreme 

 ventral portion of the median part is provided with a strong rounded 

 knob, while the middle of the median portion is decidedly roughened within. 



The type, Cat. No. 338240, U. S. N. M., measures from umbone to 

 base 12.2 mm.; from the anterior to the posterior edge, 12.3 mm.; di- 

 ameter, 13.4 mm.; palets, length 7.2 mm.; diameter 2.5 mm. 



