C. lewisi is the smallest species assigned to Camarasaurus; despite representing a very old individual, the holotype specimen is roughly 26% smaller than C. supremus, with a humerus 101.8 centimetres (40.1 in) long.

[1][2] The possible C. lewisi specimen SMA 0002 also represents a fully mature individual, but is exceptionally small, with a humerus only 70.5 centimetres (27.8 in) long.

Specimens of C. lentus exhibit a rotated pelvis, but the precise angle is difficult to confirm due to distortion of the bones and may be less than in C.

[2] The holotype of C. lewisi lacks a skull, but based on the specimen SMA 0002, which may belong to the species, the skull possesses several diagnostic characteristics: frontals with anterior midline projection into the nasals (1); trapezoidal supraoccipital (more expanded dorsally than ventrally) (2); lateral spur on the dorsal part of the lacrimal (3); fenestrated pterygoid (4); and the large pineal foramen between the frontals (5).

[8][11] Nearly all sauropodomorphs have a skull less than 50% of the length of the femur, but several Camarasaurus specimens of varying species possess such "bobbleheaded" proportions, which may be due to individual variation or sexual dimorphism.

The specimen was found in the Brushy Basin Member of the Morrison Formation, in Mesa County, Colorado.

The genus name means "perpendicular lizard" (from Ancient Greek: κάθετος, romanized: káthetos, lit.

The specific name honors Jensen's mentor Arnold David Lewis, the preparator of the Harvard Museum of Natural History.

[9] In 1996, John S. McIntosh and colleagues synonymized Cathetosaurus with Camarasaurus, noting that most of the differences proposed by Jensen pertained to the maturity of the specimen, but retained C. lewisi as a distinct species;[2] this assignment was followed by later reviews of sauropod taxonomy.

[9] It consists of a partial skeleton lacking the skull, the still present occiput probably not having been collected due its poor quality.

[21][3][22] Another unpublished study by Tschopp, Mateus, and colleagues found a third specimen, GMNH-PV 101, to form a clade with SMA 0002 and the C. lewisi holotype.