Certainly a Thracioidean and very likely within the genus Periploma but Cochlodesma is a valid possibility as well. The chondrophore and overall shape fit. Are the valves equal? Is there a posterior shell gape or overlap of one valve over the other? Historically Cochlodesma was placed by some as a "subgenus" within Periploma. If you had both valves you could have checked for a lithodesma.