296 



University of California Publications in Zoology [Vol. 22 



a further deposition of nacre on its inner surface, and in the region 

 of the umbone a secondary calcareous plate, which conceals some 

 of the earlier lines of growth, is deposited by the overlapping mantle. 

 In describing and referring to a shell so complicated in structure 

 as this one, the nomenclature employed necessarily becomes somewhat 

 involved. In order to render further discussion as lucid as possible, 

 a ready key to the terminology employed is furnished in figure A. 

 It will be observed that the three lobes of the shell are denoted 

 respectively as anterior, median, and auricle. The second of these 

 has been further subdivided into anterior median, middle median, 

 and posterior median parts. 



Pig. A. Interior and exterior views of the shell of Ter 

 ology adopted. 



3, showing termin- 



In interior view the salient features are the specialized dorsal 

 and ventral articulations, and the broad, flat process extending free 

 ventrally from the inside of the dorsal knob, the apophysis, the 

 function of which is probably to yield visceral support. 



Age and Depostion of Ridges 



Morphologically the cutting ridges on the shell of Teredo are 

 exaggerated lines of growth. Their deposition must be a function 

 of two variables: a physiological tendency to deposit ridges with 

 what seems to be an approximate periodicity, and a direct dependence 

 of the activity of the animal upon conditions of food, salinity, and 

 temperature. That is to say, the ridges on the shell are a record, 

 not alone of the age of the animal, but also of the favorable or un- 

 favorable conditions under which it has lived. To separate the 

 effects of these two factors is a highly complicated problem. 



