Joshua Kirby died on June 21, 1774 and is buried at St. Anne’s, Kew.
His gravestone is no longer especially legible, and my pictures certainly don’t help.
Here is my attempt at a transcription.
Joshua Kirby FRS-AS/ died 21st June 1774 Aged 58 / Sarah his wife / died [ ] August 1775 / Aged 57 Years / William Kirby / Son of the above / Joshua & Sarah / died 13th July 1771 / Aged 28 Years / / CGH Kirby Son of / William and Elizabeth Kirby / died an Infant / 29th October 1767 / Elizabeth wife of / William Kirby died January / 1796 Aged 49 Years
Update: Here is another view of the armorial at the top of the gravestone.