196 



each other. At the backward margin of the anterior median area the denticles dis- 

 appear, and the ridges cur\e sharply and continue across the middle median and 

 posterior median areas and the auricle as ordinar>- lines of growth. 



The interior of the shell (fig. 68) presents several features of interest. The division 

 of the shell into three major lobes is apparent from within as well as from without. 

 The forward edge of the auricle forms usually a prominent shelf by overlapping the 

 posterior median portion. The apophysis, a broad flat process extending \'entrally 

 from beneath the dorsal articulation of the valves, affords an attachment for certain 

 muscles of the foot. 



The pallets consist of a pair of calcareous structures, convex on the outer face and 

 flattened or slightly concave on the inner. Attached beneath a fleshy collar at the 

 posterior end of the animal they serve to close the entrance to the burrow when the 

 occupant is in any way disturbed. Each pallet is composed of an expanded portion 

 known as the blade, and a narrow cylindrical stalk. In the genus Teredo the blade is 

 simple and typically spatulate, while in the genus Bankia it is composed of a series 

 of cone-in-cone structures, the pallet having typically the shape and somewhat the 

 appearance of a feather. In a few forms, such as Bankia exeolpa, this fundamentally 

 compound structure is more or less concealed by an overlying membrane. 



In Teredo the distal portion of the blade of the pallet nearly always dift'ers some- 

 what from the basal portion. In such forms as Teredo navalis and T. tndliformis this 

 difference is limited merely to a greater or lesser darkening of the periostracum of the 



jJmoNt 



DORSAL 

 /tRTICl/LATION 



,V£N7«AL 

 ARTICL/LAT/ON 



Fig. 68. Interior and exterior views of the shell of Teredo, showing terminology used in description. 



distal portion of the blade; but in some other forms — Teredo samoaensis, T. diegensis, 

 T. affinis — the blade is made up of two fused but distinctly different elements, which 

 ma)' usualh' be separated intact. 



Key to the Shipworms of the P.\cific Coast and Islands 



The following key has been prepared with a view to facilitating the identification 

 of the shipworms of the Pacific Coast, and those that are known to occur in the Hawai- 

 ian, Samoan and Philippine Islands, although the list cannot be considered complete 

 as regards the latter localities. In these species the characters of the pallets alone are 

 believed to afford a practicable basis for the separation of the difterent forms, and in 

 the interests of brevity and simplicity shell characters have not been included. 



In comparing the relati\e lengths of blade and stalk, measurements should be 

 taken from the point where the blade begins suddenly to expand or to show marked 

 differentiation from the stalk. 



A key of this type is rarely infallible, and reference should be made to the more 



