Seattle has yet to sign second-rounder Austin Wilson, an outfielder from Stanford, and eighth-rounder Tyler Smith, a shortstop from Oregon State, which is still competing in the College World Series. The only other pick from Rounds 1-20 yet to agree is 13th-rounder Lachlan Fontaine, a third baseman out of Vancouver, B.C.
Nine of the players announced in the latest group are pitchers. Besides Horstman and Littell, they include Eddie Campbell (15th round), Paul Fry (17th), Troy Scott (18th), Tommy Burns (22nd), Will Mathis (25th), Tyler Wright (26th) and Michaelangelo Guzman (31st).
Also signing were infielders Lonnie Kauppila (16th round), Zach Shank (28th) and Jordan Cowan (37th), outfielder Ian Miller (14th) and catchers Daniel Torres (20th) and Kyle Petty (23rd).
Horstman and Wright -- a 22-year-old out of the University of Arkansas -- were assigned to the Class A Everett squad, while the rest will report to Rookie League ball in Arizona.
The Mariners have until July 12 at 2 p.m. PT to sign players selected in the 2013 First-Year Player Draft.
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB as well as his Mariners Musings blog. This story was not subject to the approval of Major League Baseball or its clubs.