The record of fossil turtles from the Cretaceous andCenozoic of Asia and North America is very rich, including several lineagesof cryptodiran turtles. Here we survey the shell bone histology of twoimportant closely related groups of stem trionychians, the Adocidae andNanhsiungchelyidae, which have representatives in both Asia and NorthAmerica. All studied taxa show shell bones in which the diploe is framed bywell-developed cortical compact bone layers. Taxa of both groups alsoexpress external regular surface sculpturing of their shell bones, and inthe case of the nanhsiungchelyid genus Basilemys also on theosteoderms, which is also reflected in the internal histological bonestructures. Besides similarities of the regular ornamentation patterns, bothgroups share a number of microanatomical and histological characters such asthe zonation of external cortex with rather homogeneous fine-fibred interwoven structural fibres (ISF) inthe more internal zone and a dominance of vertically oriented fibres in theISF and the presence of growth marks in the more external zone. On the otherhand, growth marks, i.e. lines of arrested growth, which are visible aswavy lines in thin sections, extend subparallel to the external bonesurface in adocids, but they are not parallel/subparallel innanhsiungchelyids. Thickness and structure of bone trabeculae in thecancellous interior regions depends on the shell bone thickness of theindividual samples. The internal cortices of all taxa except the NorthAmerican samples of Adocus usually consist of parallel-fibred bonethat locally grades into lamellar bone. Secondary bone remodelling is morefrequent in nanhsiungchelyids compared to adocids, and Sharpey's fibres thatextend perpendicular to the bone margins extending across subparallelgrowth marks are more commonly found in adocids. In addition, bone histologyserved to identify trionychid specimens in the adocid and nanhsiungchelyidsamples, especially as bone surface sculpturing patterns were weathered oreroded in those cases. The histological data present thus supplement thenumerous previously reported differences in external shell morphologybetween adocids and nanhsiungchelyids on the one hand and trionychids andcarettochelyids on the other.