T he fact that, in the discussion on Eotherium *, Dr. Murie, in his able summary of the Sirenia, made no mention of Prorastomus , and that it has not been noticed in any of the papers on new forms, or on derivative hypotheses of the order, which have appeared since 1855, begat a misgiving that the characters adduced in our Quarterly Journal for that year† might not have been deemed conclusive of the Sirenian nature of the Jamaica fossil. I therefore submitted it to a fresh scrutiny, and, endeavouring to expose more of it, was gratified by finding that further chiselling of the matrix (a hard grey limestone) exposed so many additional characters as, in connexion with the former evidence of a more generalized structure in this, perhaps, geologically oldest known Sirenian, to lead me to deem them worthy of notice. As the fossil skull is now in almost the state of completeness of that of the Felsinotherium Forrestii , I accordingly submit three views, corresponding with those illustrating the instructive memoir by Capellini in the Transactions of the Institute of Bologna‡, with a conviction that they will prove acceptable materials towards the solution of the problem of the origin and course of modification and variation of the Sirenian type of the mammalian class. The chief additions to the characters of Prorastomus , such as were shown by the reduced figures given in plate xv. vol. xi. of the Quarterly Journal of the Geological Society (1855), are of the base and roof of the