Ark. 1903. North American Plesiosaurs — Williston. 5 



suggestions, and for the communication of photographs; and to Prof. 

 II. F. Osborn for kind favors. 



Seeley* 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 iKKyt proposed the two sub-families: Poly- 

 cotylina. for those with broad epipodial bones; and the Plesiosaurina 

 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 P/iosai/n/s, a dicranopleuran, have single-headed ribs through- 

 out. Polycotylus 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, 6r without 

 such a crest; the palatines widely separated or broadly contiguous; 

 the supraoccipitals paired orsingle(P). The neck may include as few 

 as thirteen vertebrae or as many as seventy-two, the vertebrae 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 vertebrae may be no longer than the anterior cervi- 

 cals or much elongated; all the vertebrae 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. 1S92. 151. 

 t American Naturalist. 1SS7. 564. 



