MONOGRAPH OF AMERICAN SHIPWORMS. 17 



and extends at its anterior margin over about half of the posterior 

 edge of the median portion. It is about as broad as the posterior 

 portion of the median part, and is crossed by coarse concentric more 

 or less irregularly developed threads. The posterior auricle on the 

 inside laps over tbe posterior median portion but does not form a 

 consjDicuous shelf. The middle portion of the median area bears 

 a ver}' strong knob at its ventral termination on the inside. A 

 moderately strong knob also marks the umbone, from the inner side 

 of which a narrow curved sickle-shaped blade extends downward 

 over three-fourths of the distance between the umbone toward the 

 ventral knob. The anterior portion is separated from the anterior 

 median portion by a straight line. Pallets with a stalk a little more 

 than twice as long as the blade. The stalk is rounded and gently 

 sinuously curved. The blade is oval, half of its length solid, the 

 distal half marked by a series of grooves which denote segments 

 on the inside. The outside is covered by a thick, solid periostracum. 



The above description is based on one of Jeffreys's types, Cat. No. 

 194267, U.S.N.M., which he states came from drift West Indian 

 cedar {Cedrella odorata)^ taken at Leith, Scotland. 



The shell measures: Height, 5.1 mm.; length, 5.1 mm.; diameter, 

 6 mm. The pallet measures: Length, 6.7 mm., length of stalk, 4.4 

 mm. ; width of blade, 1.7 mm. 



This species resembles closely Bankia hrasiliensis, from which it 

 differs in having the dental ridges of both the anterior part and the 

 anterior median portion much more widely spaced, much stronger and 

 decidedly less numerous than in hraziliensis. It is quite possible that 

 this may belong to the West Indian region, but the mere occurrence 

 in West Indian drift cedar on the coast of Leith of course does not 

 establish this. 



Genus TEREDO Linnaeus. 



1758. Teredo Linnaeus, Syst. Nat., ed. 10, p. 651. 



In this genus the pallets are either paddle or spoon shaped. They 

 may be distally cupi3ed or not, or they may even bear a calcareous 

 knob at the terminal portion. 



Type. — Teredo navalis Linnaeus. 



Subgenus Teredo Linnaeus. 



1758. Teredo Linnaeus, Syst. Nat., ed. 10, p. 651. 



Teredo has the pallets paddle-shaped, with a decidedly cup-shaped 

 depression at the distal end. The distal portion is covered with a 

 dark periostracum, which terminates in the shape of two lateral 

 horns. 



Type. — Teredo navalis Linnaeus. 



