IV. A letter from the Reverend Mr Morton, A. M. and S. R. S. to Dr Hans Sloane, S. R. Secr. containing a relation of river and other shells digg’d up, together with various vegetable bodies, in a bituminous marshy earth, near Mearsashby in Northamptonshire: with some reflections thereupon: As also an account of the progress he has made in the natural history of Northamptonshire