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 Pliosaurus, 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 
Phosaurs 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 1 know from the 
Fort Benton deposits of Kansas; there are none certainly referred to 
this epoch from other regions, though Avimosaurus grandis Leidy 1s 
probably of this horizon. The only one of these hitherto described 
is Zrinacromerum bentonianum Cragin, a long-headed form with long 
mandibular symphysis and short neck, a form indeed approaching, 
possibly identical with Dolichorhynchops. 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 
Cimoliasaurus or Brimosaurus. A third form is much smaller, about 
the size of Dolichorhynchops osborni, with short neck. The episternum 
is shown in Fig. g and the cervical vertebrae and humerus in PI. 
XXVIII. I suspect that it belongs in 777nacromerum, though smaller 
than the type species. I have called it provisionally Trinacromerum 
anonymum n. sp. From all these forms the one described may be 
at once satisfactorily distinguished by the entire absence of infra- 
central vascular foramina. | 
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