Utanobori specimens of Desmostylus stored in AIST, Geological Museum, Tsukuba, Ibaraki, Japan were excavated from the Middle Miocene Tachikaraushinai Formation at Esashi Town (former Utanobori Town), Hokkaido, Japan. Some specimens of them was not formally described, and these specimens were described in this paper. All the 1st to 7th Utanobori specimens were considered to be D. hesperus, based on the morphology and geological age. Regarding the 3rd Utanobori specimens studied in this paper consisting of several specimens (GSJ F07745-4, GSJ F07745-6, GSJ F07745-7, GSJ F07745-8, GSJ F07745-13, GSJ F07745-14), they were collected within an identical rock. All of the 3rd Utanobori specimens showed nearly same growth stage and did not include overlapped bone elements. Thus, they were highly probable from an identical individual. Comparing stylohyoid, incisor, tibia of the 3rd Utanobori specimens with other corresponding specimens of D. hesperus (the 1st Utanobori specimen; GSJ F07743 and the Keton specimen; UHR18466), the stylohyoid of the 3rd Utanobori specimens was much delicate comparing to that of the 1st Utanobori specimens, and the occlusal surface of incisor was worn in different manner from that of the 1st Utanobori specimens. The tibia of the 3rd Utanobori specimens did not have distortion like those of the 1st Utanobori and Keton specimens. These morphological variations showed some intraspecific-variation of D. hesperus or difference in preservation state.
The 8th Utanobori specimens described previously are re-described, because of misidentification of the side and direction of the patella and osteologically insufficient description and discussion of the humerus. The right humerus (OME-U-0170) is more than 525 mm in length, and the left patella (OME-U-0171) is 112 mm in maximum thickness. The body length of an adult male Desmostylus is estimated at 387 cm and the weight at about 3.5 t from the largest humerus. The patella about 50 percent thicker than that of an Asiatic elephant suggests a larger moment arm of the knee extension, which proves that Desmostylus had a lateral-type limb posture.
In the appendix, the cranial morphology of the 1st Utanobori specimen is reconsidered based on addition of specimens for comparison.