RORQUALS IT i 



EXTINCT BAL^NIDS 



There are three important facts with regard to the 

 extinct representatives of the whalebone whales. 

 Firstly, none are known from an earlier period than 

 the Miocene ; secondly, the earliest forms appear to 

 be Balaenopterids ; and lastly, the more ancient whales 

 were not larger than existing forms. On the contrary, 

 this is a group which has increased considerably in 

 size. 



One of the best known forms, as it is represented 

 by a nearly complete skeleton, is the Miocene and 

 Pliocene Plesiocetus. P. C2ivicri was a smallish whale, 

 not more than 2 1 feet long, and distinctly belongs to 

 the Balsenopterid type. The chief interest attaching 

 to this whale is the length of the frontal, so very 

 abbreviated in other recent whales, and the share 

 which the parietals take in the formation of the roof 

 of the skull. In the living whalebone whales these 

 bones are covered in by the supra-occipital. Like 

 the modern Bal&noptera this genus comprises both 

 large and small species. Cope states that Plesiocetus 

 brialuwnti was some 60 feet in length. 



o 



Mesoteras of Cope was thought by him to be some- 

 what intermediate between Balcenoptera and Balcena. 

 It has "the characters of the Finner whales (>al<zn- 

 optera\ with the narrow maxillary bones of the true 

 Balana." It is a large species, with a skull 18 feet 

 long, evidently so far a Baltzna. There is an "enor- 

 mous thickening of the superciliary part of the frontal 

 bone." The existing genera are also known as fossils. 



