Zynga will acquire A Bit Lucky, a digital gaming studio known for Facebook-based titles like "Lucky Train."

Terms of the deal, announced on Monday, were not disclosed.

Zynga said more than 20 A Bit Lucky employees, currently based in San Mateo, California, will join Zynga's San Francisco division.

The acquisition reflects Zynga's shift toward so-called mid-core offerings - titles that sit between highly produced, packaged games and relatively basic "casual games" like Words with Friends and FarmVille that for years have been the bread and butter of Zynga's business.

The deal comes shortly after Zynga hired John Tobias, a co-creator of the "Mortal Kombat" series, to work on a new mid-core title, Zynga executive Bill Jackson noted in a blog post on Monday.

The game publisher is diversifying its product line at a time when usage numbers for even its most popular online games like "FarmVille" and "CityVille" are trending downward, prompting questions about whether casual gaming will remain popular in the months and years to come.

The company is also locked in litigation with competitor Electronic Arts, which sued Zynga last month saying it unfairly copied elements of EA's "Sims" game.

Zynga then accused EA on Friday of breaching a settlement in which Zynga agreed to restrictions on how it could try to hire EA employees. Those details were supposed to be kept out of public view, but Zynga inadvertently disclosed them.

Zynga's shares closed on Monday at $3.08, down 3.1 percent for the day and off almost 70 percent from its $10 December IPO price.