1922] Miller: Variations in Teredo navalis in San Francisco Bay 295 



THE SHELL OP TEREDO 

 Description 



The shell of a larval Teredo at the < im<» of its settlement on a 

 submerged timber is that of a typical bivalve, nearly circular in 

 outline, with prominent umbones. The valves arc equal and ex- 

 tremely convex, so that together they have a subglobular form, this 

 being characteristic of teredine larvae. A few lines of growtli are 

 indistinctly visible. 



Very soon after settlement and the commencement of boring, 

 however, marked changes become evident. Details of the meta- 

 morphosis are reserved for a later paper ; suffice it to state here that 

 the shell grows more rapidly at the ventral and posterior margins 

 than anteriorly, so that the median portion becomes the most promi- 

 nent part of tlie shell, while at the same time a pronounced wing or 

 auricle is developed posteriorly, on the inner surface of which the 

 large adductor muscle is attached. Because of the peculiar method 

 of growth, a permanent gape is left between the valves both in front 

 and behind ; through the former of these protrudes the specialized 

 subcircular foot, while through the latter the elongated body extends 

 posteriorly. 



A few days after the commencement of growth in the wood, the 

 shell has assumed the form of the adult, in that each valve has a 

 distinct anterior, a large median, and a posterior lobe. A knob or 

 secondary point of contact early develops at the ventral edge of the 

 median portion, so that the shells are able to rock on the original 

 dorsal and secondary ventral articulations as fulcra, the anterior 

 and posterior adductor muscles now opposing each other in action. 

 The ridges on the anterior lobe are parallel to its margin, and meet 

 at nearly right angles the prominently toothed ridges of the anterior 

 part of the median lobe. Growth now occurs by addition of succes- 

 sive increments to the margin in such a way that the newly estab- 

 lished shape of the shell is but little altered; each ridge on the 

 anterior lobe meets at a right angle a ridge of the median lobe, which 

 in turn is continuous around the ventral margin with a more or less 

 prominent line of growth on the posterior portion. There is at the 

 same time an increase in the thickness of the entire shell, owing to 



