PROF. OWEN ON PEOEASTOMUS SIEENOIDES. 567 



No doubt both Haliiherium and Felsinotherium foreshow the molar 

 pattern of Hippopotamus ; but Prorastomus shows that of Lophiodon 

 and Tapirus, to which Manatus still adheres rather than to any artio- 

 dactyle type of molar. 



Hence, permitting oneself to indulge in the easy task of feigning 

 hypotheses, one might suggest that both Ungulates and Sirenians 

 diverged at the same remote period from a more generalized (Cre- 

 taceous?) mammalian gyrencephalous type — and that the marine 

 Herbivora, in the course of long Eocene and Miocene eons, became 

 subjected to conditions and influences calling out, respectively, ana- 

 logous modifications of molars, in one tending to an artiodactyle, in 

 other to a perissodactyle character of such teeth. 



Prorastomus, by its more generalized dentition and shape of brain, 

 represents a step nearer such speculative starting-point than does 

 any hitherto discovered extinct or existing Sirenian. This it is 

 which gives so great an interest to the "West-Indian Tertiary genus, 

 and excites so strong a desire to learn more of the conservable, 

 petrifiable parts of the extinct species. 



In the instructive memoir "On the West-Indian Tertiary Fossils"*, 

 by R. J. Lechmere Guppy, Esq., E.L.S., E.GLS., these fossils, chiefly 

 shells, forwarded to him by Mr. Yendryes, of Jamaica f, are referred 

 to Miocene Tertiaries ; and it does not appear that any of these speci- 

 mens were obtained from the compact limestone-bed of Prorastomus 

 underlying the general (Miocene?) " carious limestone," at Ereeman's- 

 Hall Estate, between the parishes of St. Elizabeth and Trelawney.J 



Eurther search in the river-course of that locality might be re- 

 warded by the discovery of more parts of Prorastomus in the com- 

 pact limestone there. Any shells or other invertebrate fossils in 

 that formation would be most acceptable, as being likely to afford a 

 clew to its age. 



EXPLANATION OF THE PLATES. 



Plate XXVIII. 

 Fig. 1. Side view of the skull of Prorastomus sireno'ides. 



2. Upper view of the skull of Prorastomus sireno'ides. 



3. Under view of the skull of Prorastomus sireno'ides. 



( These figures are of half the natural size.) 



Plate XXIX. 

 Fig. 1. Side view of retained molars, left side, upper jaw: the symbols are ex- 

 plained in the text. 

 2. Grinding surface of right upper penultimate molar, m 2. 

 o. Plan of the molar series, right side, upper jaw: the lower numbers in- 

 dicate their position from before backward : the upper numbers 

 symbolize the true, or three last, molars. 



4. Eight-side view of the symphysial part of the mandible and remnants of 



teeth therein. 



5. Hind view of part of the atlas vertebra. 



(These figures are of the natural size.) 



* Geological Magazine for September and October 1874. t lb. p. 404. 



| Quart. Journ. Geol. Soc. vol. xi. 1855, p. 541. 



