The animal on the cover of Programming Atlas is a black murex snail shell (hexaplex nigritus). The black murex is found off the gulf coast of California and Mexico. As the black murex ages, its shell turns from white to predominately black. However, pure white or black shells are very rare. Mature black murexes are about 6 inches (15 cm) long.
Black murex snails are carnivorous gastropods. Their diet is composed of bivalve mollusks, including oysters, clams, and sea anemone. Gastropods kill their prey by various means, including smothering, tearing, or boring into the shell by using an acidic mucus to weaken the outside surface.
The murex snail played a crucial role in the culture and trade of the ancient Phoenicians. They crushed the murex in order to extract a purple-red secretion used to dye fabric. It is estimated that some 10,000 snails were needed to dye one toga. As a result, only royalty could afford the precious dye for clothing. When the dye was combined with silk imported from China, the purple garments were worth more than their weight in gold. Purple has since been equated with royalty, but the red of papal robes and the blue in the flag of Israel are also derivative of murex snail dye.
The cover image is from Johnson’s Natural History. The cover font is Adobe ITC Garamond. The text font is Linotype Birka; the heading font is Adobe Myriad Condensed; and the code font is LucasFont’s TheSans Mono Condensed.