MONOGRAPH OF AMERICAN SHIPWORMS. 41 



terior median portion is very broad and crossed by the dental ridges 

 which are strongly denticulated and separated by mere lines of 

 growth. These join those of the anterior area at almost a right 

 angle. The middle median area is scarcely concave and crossed by 

 rough lines of growth. The posterior median area is a little nar- 

 rower than the anterior and middle median area combined, and 

 crossed by rather rough lines of growth, which are of irregular 

 strength. The posterior area forms a very narrow auricle, which 

 is distinguished from the posterior median area by an impressed 

 groove, and is marked by the continuations of the rough lines cross- 

 ing the posterior median area. The interior is bluish white, the 

 anterior and the posterior areas being joined by a slightly raised 

 cord. The middle median area forms a broad groove, in the ven- 

 tral margin of which the strong knob is situated. The auricle is very 

 narrow and not diiferentiated from the posterior median area. It 

 shows the lines of growth by transmitted light. On the under side of 

 the umbonal knob the broad, somewhat twisted blade descends 

 through half the length of the shell. The pallets are spoon-shaped. 

 The stalk is short, slender, and somewhat twisted. The exterior 

 is roughly scaly, marked with concentric lines at its terminal portion. 

 The inside is smooth and shows the stalk passing through the center 

 of the blade. The distal portion is marked by a heavy periostracum 

 showing lines of growth. 



The type. Cat. No. 36046a, U.S.N.M., was collected by Mr. Hemp- 

 hill at Cedar Keys, Florida. It measures : Height, 7.5 mm. ; length, 

 7.5 mm. ; diameter, 7.5 mm. The pallet measures : Length, 5.5 mm., 

 of which 1.7 mm. go to the stalk; diameter, 2.5 mm. 



The present species recalls Teredo {Psiloteredo) sigerfoosi but 

 can be at once diiferentiated from it by the very wide anterior me- 

 dian area. We have seen additional specimens from San Andrews, 

 Florida, collected by Mr. C. S. Smith on October 5, 1896, and also 

 several odd valves taken by Prof. Sigerfoos at Eivers Island, Beau- 

 fort, North Carolina, on November 16, 1909. 



TEREDO (PSILOTEREDO) KNOXI Bartach. 

 KKOX SHIPWORM. 



Plate 29, fig. 2 ; plate 34, fig. 2. 



1917. Teredo knoxi Baktsch, Bull. No. 28, Public Works of the Navy, p. 

 47, 7 figs. 



Shell small, white. The anterior portion with the usual sinus, and 

 the anterior ventral margin prolonged into a decided point. The 

 sinus is covered by a strong reflected callus, from the dorsal margin 

 of which the dental ridges bend suddenly downward and backward. 

 Of these dental ridges, 50 are present in the type. They slope a little 



