geometry - Why is '$\frac{r^2\pi}{d^2}=\frac{\pi}{4}$'? (a.k.a. the ratio between a square and a circle, $a=d$) - Mathematics Stack Exchange
Bulletin of the British Museum (Natural History). Zoology . Supplement.. CARDIGAN BAY RECENT FORAMINIFERA 193 Genus BUCCELLA Anderson, 1952 Buccella frigida (Cushman) (PI. 18, fig. 13; Text-fig. 42, nos 1-5) Pulvinulina