564 PE0F. OCTETS ON PEOEASTOMUS SIEENOIDES . 



and now extinct genus thus adheres more closely to the diphyodont 

 dental type, and not only in number, but in the different proportions 

 and configurations of the teeth, whereby the three true molars are 

 more plainly differentiated from the antecedent premolars. The 

 teeth at the fore part of the jaws, viz. incisors and canines, retain 

 the common type as to number and kind in Prorastomus, and have 

 not been subject to so great a degree of suppression, or of individual 

 excess of development, as in existing Sirenians. 



In Felsinotherium the molar series appears to be represented by 

 ■J^. Of these teeth five molars remain in place on the right side of 

 the upper jaw, and three molars on the left side. In the lower jaw 

 so much only of the alveolar part is preserved as to retain three 

 molars in each ramus. 



They increase in size, gradually, from before backward, retain 

 the general type as respects roots and enamelled crown of those of 

 Manatus and Prorastomus, and show an outer and an inner indent 

 in the least-worn hindmost molars, indicative of the bilobed struc- 

 ture ; but each lobe is subdivided, as in Hippopotamus, into two 

 parts by a longitudinal wavy cleft, and the grinding-surface of the 

 enamel-tissue is further extended by shallow accessory folds. The 

 molars of Prorastomus had not this complexity : they were more like 

 those in Manatus, and yet had not the fore-and-aft sides of the 

 primary lobes so indented, whereby in Manatus, when the crown 

 has been worn as low as in the eighth grinder of Prorastomus, the 

 enamel borders, especially of the transverse valley, are more or less 

 wavy. "When worn down lower the resemblance in Manatus to the 

 similarly worn molars of Prorastomus becomes greater. 



The molars of Prorastomus are more like miniatures of those of 

 Dinotherium than of any Sirenian. 



As compared with the skull of Manatus americanus, the orbit is 

 less advanced in position. In Prorastomus the extent from the hind 

 border of the orbit to the fore end of the prsemaxillarigs is equal to 

 that from the hind border of the orbit to the occipital ridge. The 

 maxillo-prsemaxillary or facial part of the skull is also deeper as 

 well as longer in Prorastomus. The alveolar part of the maxillary 

 descends vertically below the prominent lower border, or floor, of 

 the orbit for about an inch. In Manatus the horizontal plate of the 

 maxillary supporting the malar floor of the orbit is much more ex- 

 tensive, and the vertical part of the maxillary curving down to the 

 outlets of the sockets of the three anterior teeth is less than a quarter 

 of an inch in extent. In Prorastomus, also, the penultimate and two 

 antecedent molars are beneath the orbit, and the four anterior pre- 

 molars are in advance of that cavity ; but not any of the molar series 

 are in advance of the orbit in Manatus. 



If the somewhat thickened border of the zygomatic plate of the 

 squamosal (PI. XXYIII. fig. 1, 27) is natural and the fractured and 

 dislocated plate of bone beneath (ib. ») belongs not to that part of the 

 zygoma, then the squamosal part of the arch is much less deep, as 

 it unquestionably is much less thick, than in Manatus, and the 

 zygomatic arch must have shown more normal or ordinary mammalian 

 proportions in Prorastomus. 



