Apr. 1903. NorrH AMERICAN PLESIOSAURS—WILLISTON. 5 
suggestions, and for the communication of photographs; and to Prof. 
H. F. Osborn for kind favors. 

Seetey* has proposed to divide the plesiosaurs into two chief 
groups, the Dicranopleura, including those forms with double-headed 
ribs in the cervical region, both long-necked (Dolichodeira) and short- 
necked (Brachydeira); and of which singularly no certain representa- 
tives have been discovered in America; and the Cercidopleura, those 
with single-headed ribs, also including both long-necked and short- 
necked types. Cope in 1887+ proposed the two sub-families: Po/y- 
cotvline for those with broad epipodial bones; and the //eszosaurine 
for those with elongated epipodial bones, of which there are no 
certain representatives in America. But objections may be urged 
against both of these classifications. Certain forms very closely 
allied to Pliosaurus,a dicranopleuran, have single-headed ribs through- 
out. Polycoty/us is a short-necked type, with single-headed cervical 
ribs, and it seems almost certain that certain long-necked forms that 
should be widely separated have also broad epipodial bones. 
Nevertheless, I feel pretty confident that the final classification of 
the Plesiosauria will include three or four distinct families and twenty 
‘or thirty well-defined genera. There is scarcely a group of extinct 
reptiles, ‘unless it be the Dinosauria, which offers more divergent 
characters than do the plesiosaurs. The skull may be long and 
slender or short and broad; the teeth irregular in size and large, or 
small and nearly uniform; the prefrontals and postorbitals separated 
or suturally united; the parietals with a high thin crest, or without 
such a crest; the palatines widely separated or broadly contiguous; 
the supraoccipitals paired or single(?). The neck may include as few 
as thirteen vertebre or as many as seventy-two, the vertebra all 
very short or the posterior ones elongated; the ribs single or double- 
headed; the arches anchylosed to the centra or suturally free through- 
out life. The dorsal vertebra may be no longer than the anterior cervi- 
cals or much elongated; all the vertebra may have conspicuous 
vascular foramina below or be without them; the diapophyses may 
be much elongated and situated low down, or shorter and situated 
high up; the vertebral spines elongated or short. In the pectoral 
girdle there may be a long epicoracoidal process; the clavicles and 
episternum either present or absent. The epipodial bones are two 
* Proc. Royal Soc. Lond. 1892, 151. 
+ American Naturalist, 1387, 564. 
