The phrase "ventral surface" refers to the underside or belly side of an object or organism. It is the opposite of the "dorsal surface," which refers to the upper or back side. Full definition
Males tend to pull up some bright yellow on their anterior ventral surfaces as they grow. (nps.gov)
Its head's ventral surface lacks the numerous prominent furrows of the related rorquals, instead bearing two to five shallow furrows on the throat's underside. (en.wikipedia.org)
The fish lie on the side or along the ventral surface and lift the head off the ground, rotate it over the body towards the tail end and jump away. (sciencedaily.com)