Given the title of its sixth release, it's fair to wonder if Mastodon is hinting at 2011's The Hunter or its back catalog.

Many of the tracks on Once More 'Round the Sun dig into the band's seemingly inexhaustible bag of monstrous riffs and wonderfully fractured motifs.

That said, as a collective, they unapologetically explore the more polished and accessible songwriting and performing craft found on The Hunter.

This set marks a fork in the road where Mastodon evolves once more, to cross over from metal's angular, sludgy power to adrenaline-fueled, hook-laden, hard rock.

The album was produced by Nick Raskulinecz, best known for his work with Foo Fighters and Rush.

The sound Mastodon pursues here draws inspiration from the '70s, without remotely being an exercise in nostalgia.

There is one notable exception; it's deliberate and obvious: "The High Road" boasts unapologizing Thin Lizzy worship, albeit ambitiously updated.

(Who better?) Its verse/riff structure weds Lynott's rhythmic sensibility to Mastodon's dynamic aggression.

The anthemic chorus melody and harmonies, and twinned lead guitar roar, were trademarked by Lizzy's Brian Robertson and Scott Gorham long ago.

"Chimes at Midnight" is intense, fueled by a mammoth chugging riff.

It lets the "drop D" freak flag fly, with a near-shouted vocal, harmonic chorus, and spacy six-string interludes.

"The Motherload," with its swaggering guitar heroics, is a wound-out yet nearly hummable prog melody, with a relentless bass and snare attack.

"Aunt Lisa," with its knotty guitar intro, contains processed vocals, a series of rising and falling key changes, and the Coathangers guesting -- cheerleader style -- in a chanted vocal chorus à la Faith No More's "Be Aggressive!" There are also some substantive guitar pyrotechnics in the extended solos in "Halloween" and "Ember City," which, due to their imagination and focus, add dimension to them as songs.

"Diamond in the Witch House" is a sprawling, nearly eight-minute closing jam.

Neurosis' Scott Kelly and his menacing growl guest as it lumbers, trudges, and lurches ever forward (longtime fans will likely dig this).

Once More 'Round the Sun furthers what Mastodon began on The Hunter: expanding their music past metal's rigid borders -- toward an integrative sound that doesn't leave metal out.