MOiSrOGRAPH OF AMERICAN SHIP WORMS, 33 



KEY TO THE AMERICAN SPECIES OF TEREDORA. 



Pallets : 



Nail strongly ribbed panamensis. 



Nail obsoletely ribbed thomsoni. 



The pallets of vincentensis are uukuowu. 

 Shell : 



Anterior portion of median part moderately broad thomsoni. 



Anterior portion of median part very narrow. 



Height more than 10 mm vincentensis. 



Height less than 5 mm panamensis. 



TEREDO (TEREDORA) THOMSONI Tryon. 

 THOMSON SHIPWOEM. 



Plate 26 ; plate 34, fig. 4. 



1863. Teredo tlionisonii Tryon, Proc. Acad. Nat. Sci. Philadelphia, ser. 2, 

 vol. 7, pp. 280, 281, pi. 2, figs. 3-5. 

 Shell large, white. The extreme anterior area with the usual sinus 

 and heavy callus, which forms a strong projecting cord at the edge of 

 the shell. From this sinus the dental ridges bend first downward and 

 then pass backward in an even curve, spreading out slightly fan 

 shaped. They are about half as wide as the spaces that separate them 

 at the posterior termination in the later portion of the shell, but in 

 the first fourth they are not more than one-fourth as wide. The 

 anterior callus covers up the early ones, but 68 are present in the 

 specimen figured. The anterior part is separated from the posterior 

 median part by a straight line. These ridges slope abruptly on the 

 dorsal side, and gently on the ventral which is about twice as broad. 

 They are finely denticulated on their free margin. The anterior 

 median part is crossed by dental ridges that join those of the anterior 

 part at a little more than a right angle. These ridges are separated 

 by rather broad lines, and bear numerous fine denticles at their free 

 margin. The median portion of the middle part is concave and 

 crossed by coarse lines of growth. The posterior portion is much 

 wider than the anterior, and is marked by irregular lines of growth. 

 The posterior part forms a small auricle which is very obliquely 

 placed and strongly constricted at its junction with the posterior 

 median portion. Interior bluish white. The anterior and median 

 portions are joined by a somewhat thickened area, while the middle 

 portion of the median part is a rough groove that extends apparently 

 from the umbone to the strong knob at the base. The auricle is 

 placed on the posterior median portion in such a way that it looks 

 almost like a pallet attached in the middle to the posterior median 

 part, about as much projecting inward as projects outward. From 

 the under side of the knob at the umbone a strong blade, which is 

 broad and becomes expanded basally, extends for half or more of the 

 length of the shell downward. The pallets have a subcylindric 



