Mammillaria tetrancistra

Mammillaria tetrancistra is one of only three Mammillarias that grow naturally in California. The plant considered pretty difficult in culture and usually grafted.
In the picture this plant thrives in harsh conditions of Joshua Tree National Park desert. The plants are small there and without bright fruit they blend into the environment.
 The picture is taken in May when many other cacti are blooming. Mammillaria tetrancistra blooms much later, in August, responding to occasional monsoon rains that are eventually  reaching that far east.