The Sea Egg by Lucy Boston
The Sea Egg by L.M. Boston (the author of the Green Knowe series) 1967
A story about two young boys (brothers) vacationing on the Cornish coast who discover to their delight and wonder a sea egg from which hatches a triton (or a merman). Their adventures are simple, and though extraordinary they ring true to life, as do the characters of the boys.
If you are acquainted with Boston’s "Green Knowe" series you will know that she is prodigiously gifted with a poetical prose.