MONOGRAPH OF AMERICAl!^ SHIPWORMS. 37 



Shell : 



Auricle very large dilatata. 



Auricle not large. 



Anterior portion of median part very narrow sigerfoosi. 



Anterior portion of median part not very narrow. 



Auricle moderately large jamaicensis. 



Auricle small. 



Height more than 7 mm. 



Incremental lines of middle and posterior 



median part coarse stimpsoni. 



Incremental lines of middle and posterior 



median part fine tryoni. 



Height less than 6 mm knoxi. 



TEREDO (PSILOTEREDO) DILATATA Stimpson. 

 BIG-EAKED SHIPWOEM. 



Plate 28, fig. 1 ; plate 36, fig. 2. 

 1851. Teredo dilatata Stimpson, Proc. Best. Sec. Nat. Hist., vol. 4, p. 113. 

 Shell of medium size, white. The extreme anterior portion with 

 the usual sinus and reflected callus. The latter covers a considerable 

 portion. The ventral anterior portion projects forward as a decided 

 point from which the dental ridges radiate downward and backward. 

 These are separated by spaces about twice as wide as the dental 

 ridges at the posterior margin. Twenty-three are visible in the speci- 

 men figured. These dental ridges slope abruptly on the dorsal mar- 

 gin and gently toward the ventral, the ventral side being fully three 

 times the width of the dorsal. They are finely denticulated at the 

 free edge. The anterior median part is quite narrow and marked 

 by strongly denticulated ridges, which are separated by mere im- 

 pressed lines. The middle median part is not strongly depressed, 

 but crossed by strong cords, which are the nondenticulated continua- 

 tion of the dental ridges of the anterior median part and between 

 these are finer threads, which coincide with the heavier in dispo- 

 sition. The posterior median portion is about one and a half times 

 as wide as the anterior and middle median portion and appears 

 wavy, the crest of the Avaves corresponding with the stronger ridges 

 described for the middle median portion. The posterior part forms 

 a huge auricle which is almost as wide as the middle part. It is 

 thin at the free margin and slightly reflected outward. It extends 

 prominently upward and has a strong incised margin near the 

 umbones. It is marked by a series of concentric feeble wave-like 

 elements. The interior is bluish white, the suture of the anterior 

 and median part being a mere raised thread, while the middle part 

 is marked by a groove, the extreme ventral portion bearing the 

 prominent knob. The auricle fuses with the posterior median part 

 and does not in any way project over it, but it seems rather as if 

 the median part covered the aui-icle slightly with a tliin callus. The 



