PK0E. OWEN ON PRORASTOMUS SIRENOIDES. 559 



42. On Peorastomus sirbxoides (Ow.). — Part II. 

 By Prof. Owen, C.B., F.R.S., P.G.S. (Read June 9, 1875.) 

 [Plates XXVIII. & XXIX.] 

 The 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 Quar- 

 terly Journal for that yearf 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 con- 

 nexion 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 accord- 

 ingly submit three views, corresponding with those illustrating the 

 instructive memoir by Capellini in the Transactions of the Institute 

 t of Bologna J, 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 cranium, the zygomatic arches, the hind half of the 

 mandible, with the articular surface of the condyle, and the major 

 part of the atlas vertebra. 



To the description of the occipital surface (he. cit. p. 542, pi. xv. 

 fig. 1) may be added, that no trace of the sutures between the super- 

 and ex-occipitals remains, the outer half of such sutures being pre- 

 sent in a Manatus americanus with six of the molars in place on each 

 side of both jaws. This gives evidence of the maturity of the smaller 

 fossil Sirenian. The paroccipitals are broad, compressed from behind 

 forward, and slightly bent, with the concavity backward, as in Manatus. 

 In the concavity between the base of the par- and basi-occipitals 

 opens the praecondyloid foramen, as in Manatus and Rhytina. The 

 basioccipital has been fractured transversely a little in advance of 

 the praecondyloid foramina ; and the fore part has been pressed up- 

 ward for a line's breadth or more above the level of the hind part 

 of that bone. The length of the basioccipital (PL XXVIII. fig. 3, i ) 

 is 1 inch 8 lines ; and the suture with the basisphenoid (PI. XXVIII. 



* Proc. Geol. Soc. November 18th, 1874. 

 • t Quart. Journ. Geol. Soc. vol. xi. p. 541, pi. xv. (1855). 



J ' Memorie della E. Accadeniia delle Scienze dell' Istituto di Bologna,' Serie 

 terza, torn. i. p. 605, tav. i.-iii. (1871). 



Q.J.G.S. No. 124. 2p 



