There are a number of species of the subgenus Servatrina that need to be considered. Rosewater (1961) lumped all similar looking shells under pectinata L. 1767 and that is the name I used in the Arabian book. Huber has many of Rosewaters synonyms reinstated but he dismisses both serra and penna from consideration for Indian Ocean species. If you follow his ideas then you would use the name hystrix of Hanley. As with many pinnids I suspect that it is quite variable and I am reluctant to give a definite answer without reviewing all possible material. I am pretty sure your shells are the same as mine from southern Arabia.

Graham

Es cierto es indispensable examinar la muestra. Creo que 

podría ser Atrina serrata (G.B. Sowerby I, 1825)

Can you highlight the internal muscle imprints?

Best, Gianluigi

This is Atrina penna a young specimen of west coast of India.