The pattern is "Lady of the Lake" by Thomas and John Carey (marked) C. 1830. The design was based on engravings of drawings by Richard Westall which were illustrations in the first edition of Sir Walter Scott's "Lady of the Lake" published in 1810. 
The platter has a lovely blue tinted pearlware glaze.  Size: 21 1/4" x 15 7/8". 

    
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