The mark is partially obliterated by a blob of glaze. In full it looks like this . . .
Clément, Jérôme & Delphin were three brothers from a family of potters in Vallauris, southern France. All three worked in the family business of Jérôme Massier et Cie (Jérôme was their father's name) in Vallauris, but later worked independently.
Clément, the middle brother, born in 1845, was the most prominent of the three. He worked in Vallauris before moving to nearby Golfe-Juan, and is remembered for his lustre glazes which were later taken up by the American Weller company. He was a leading Art Nouveau figure and at the turn of the century opened a salon in Paris.
Jérôme, born 1850, and Delphin, born 1836, continued working in Vallauris. Of these two Jérôme's work is the more sought-after.
Vallauris is a pottery region in south-eastern France, near Cannes. Pottery has been made there since ancient times. The area grew as a pottery centre in the late 19th century, and many companies have flourished there since then, notably those of Massier and Foucard-Jourdan.
Modern Vallauris is lead-glazed earthenware, often highly coloured; the Mediterranean location sometimes showing its influence. Domestic tableware and cooking pots feature prominently.