Working in rows......

Most of the squares I have made so far have been done in rounds, this one is made working from side to side:

Each row includes four and a half 'shells'. The start of each row is 3 chains (counts as 1 treble crochet), 2 treble crochets are then worked in the stitch at the base of the 3 chains, skip two chains and work a double crochet in the next chain, this is a half shell. Whole shells are completed by skipping two chains and then working five treble crochets in the next chain, skip two chains and work a double crochet in the next chain. Continue across the row of chains finishing with a double crochet in the last chain.
Subsequent rows are worked in the same way. Each row starts with a half shell (double crochet in the middle treble of the last shell on the previous row), whole shells are worked by doing five treble crochets in the double crochet from the previous row and the double crochet is worked in the central treble crochet from the shell on the previous row.
I have edged it with a round of double crochets, as usual, to finish it off and make it easier to join to other squares.