404 University of California Publications in Zoology [Vol. 22 



NATURE AND CAUSES OP VARIATIONS IN THE PALLETS 



We have figured, in plate 20, series of pallets from three localities 

 in San Francisco Bay, namely, Crockett, Dumbarton and Goat Island. 

 For the location of these stations and a discussion of the ecological 

 conditions characteristic of each one, the reader is referred to our 

 earlier paper (1922, pp. 301 ff.). 



In each of the series in question, the pallet at the extreme left 

 of the illustration is regarded as approaching the type of navalis, as 

 figured, by Jeffreys (1869) in volume 5 of the British Conchology 

 (pi. liv, fig. 2). The other specimens in the series represent depart- 

 ures in greater or less degree from the type. The smaller size of the 

 Dumbarton material is not regarded as significant; our specimens 

 from this locality were secured from timbers exposed only a few 

 months, whereas specimens from the. other two localities were taken 

 when older. 



In studying variation in the pallets, it is important that physio- 

 logical factors should be separated so far as possible from purely 

 physical ones. Of the latter there are those already mentioned of 

 accident and erosion. The pallets are, from the nature of their 

 function, much exposed to unfavorable circumstances. In times of 

 stress they form the barrier against undesirable and often lethal 

 conditions. They are much subject to wear from being frequently 

 thrust into the constricted end of the burrow. While in such position 

 they may be eroded by organic acids present in the water or the 

 wood. If a foreign object, a piece of floating driftwood, for example, 

 comes violently into contact with the surface of a timber in which 

 teredos occur, the pallets are likely to be broken. 



In plate 19, figure 4, it will be seen that even a pallet four months 

 old may show very decidedly the effects of wear. In figure 5 of this 

 plate we have compared a normal and rather typical pallet from Goat 

 Island with an older pallet from the same locality which illustrates the 

 maximal effect of age and erosion, the original form of the distal 

 portion of the blade being entirely lost. The secondary calcareous 

 accretions about the stalk may be pathological; similar accretions 

 have been noted in other cases, as in plate 20, figure 6 g, but are 

 infrequent. 



