66 Field Columbian Museum — Geology, Vol. II. 



not only in these, but in other aquatic air-breathing vertebrates, such 

 as the cetacea, some ichthyosaurs and the mosasaurs, due to environ- 

 mental causes. It is true that all the Squamata show the same single- 

 headedness of the ribs, brought about by similar conditions — the lack 

 of the necessity of support of the abdominal organs by the ribs in 

 animals resting prone upon the ground, or in a medium of nearly the 

 same specific gravity as the creatures themselves. 



It is a singular fact that, in many plesiosaurs, vestiges of dicran- 

 ial ribs have been retained in the neck, though such have disappeared 

 elsewhere in the vertebral column; and this character has been 

 retained in both the long-necked and the short-necked types, such as 

 Plesiosaurus and P/iosaurus, though utterly wanting in others, such as 

 Elasmosaurus with seventy-two cervicals and the present with only 

 thirteen. Did the long-necked forms become differentiated before 

 the dicranial character was lost, and have they continued as a distinct 

 phyllum until the character was wholly lost? If so, the short-necked 

 Pliosaurs must represent a distinct branch of the order which has also 

 undergone the same change. 



The Cretaceous plesiosaurs of America, so far as known, are all 

 cercidopleural, while many of the European Jurassic forms are 

 dicranopleural. 



This is the fourth species of plesiosaur that I know from the 

 Fort Benton deposits of Kansas; there are none certainly referred to 

 this epoch from other regions, though Brimosaitrus grandis Leidy is 

 probably of this horizon. The only one of these hitherto described 

 is Trinacromcrum bcntonianum Cragin, a long-headed form with long 

 mandibular symphysis and short neck, a form indeed approaching, 

 possibly identical with DolicJiorhynchops. Another form known of 

 which a considerable part of the vertebral column is preserved at the 

 museum of the University of Kansas, is of great size, the dorsal centra 

 measuring five inches or more in diameter, with a very long neck and 

 small anterior cervicals. The specimen is from near Beloit. It 

 represents a distinct species that may provisionally be referred to 

 Cimo/iasirurus or Brimosaurus. A third form is much smaller, about 

 the size of DolicJioriiyncJiops osborni, with short neck. The episternum 

 is shown in Fig. 9 and the cervical vertebrae and humerus in PI. 

 XXVIII. I suspect that it belongs in Trinacromcrum, though smaller 

 than the type species. I have called it provisionally Trinacromcrum 

 anonxmum n. sp. From all these forms the one described may be 

 at once satisfactorily distinguished by the entire absence of infra- 

 central vascular foramina. 



