TORONTO (Reuters) - Shopify Inc <SHOP.TO> <SHOP.N> shares fell for the second consecutive day in heavy trading on Thursday after short-seller Citron Research said the stock was overvalued and criticized the Canadian ecommerce software provider's marketing practices.


"We vigorously defend our business model and stand resolutely behind our mission and the success of our merchants," the company said in a statement.


Shares of Shopify were last down 7.8 percent at $95.23 on the New York Stock Exchange, where their volume was already three times the 90-day average. They were down 7.5 percent at C$119.25 on the Toronto Stock Exchange.


Shopify has yet to turn a profit since it was founded in 2004. Its stock tumbled more than 11 percent on Wednesday after Citron said in a report that the company "oversells" the potential for profitability.


(Reporting by Solarina Ho and Jim Finkle; Editing by Lisa Von Ahn)