234 S. W. Williston — North American Plesiosaurs. 



all improbable that the validity of the generic name may be 

 eventually called into question, since there seems to be no dif- 

 ference between the teeth of this form and that described by 

 Leidy years before, from the Cretaceous, presumably Niobrara, 

 of Minnesota, as Piratosaurus. However, as the identity 

 must remain for many years, if not always, more or less doubt- 

 ful, it would be very unwise to make any changes at the 

 present time. 



The Yale specimen, presenting as it does not a few interest- 

 ing morphological and structural characters, will be fully 

 described and figured later. It comprises the larger part of 

 the skeleton, with the lower jaws, parts of the skull, teeth, etc. 

 From the study of this specimen, supplemented by other 

 specimens, clearly conspecific, the generic characters may be 

 stated as follows : — 



Polycotylus. Teeth rather slender, with numerous well- 

 marked ridges. Face with slender beak. Cervical vertebrce 

 twenty-six in. number; dorsals twenty -eight or twenty-nine, 

 inclusive of three pectorals ; all short and all of nearly uni- 

 form length. Chevrons articulating in a deep concavity / 

 all the vertebral, and especially the cervicals, rather deeply 

 concave, and with a broad articular rim. Pectoral girdle 

 with distinct clavicles, interclavicles, and interclavicular fora- 

 men / the scapulae not contiguous in the middle. Coracoid 

 with a long anterior projection, united in the middle, back of 

 the inter glenoid bar, to the posterior margin ; a foramen on 

 each side, back of interglenoid thickening. Ischia elongated. 

 Paddles, with four epipodial bones, all much broader than 

 long. 



The foregoing characters, it will be seen, are very much like 

 those already given by me for Dolichorhyn chops, and I am 

 somewhat in doubt as to the validity of that genus, or rather 

 of Trinacromerum Cragin, of which, as I suspected, Dolicho- 

 rhynchops is a synonym. The only important distinctions are 

 the deep concavity of the centra and the mode of articulation 

 of the chevrons. In none of the known species of Trina- 

 cromerum are there more than three epipodial bones, while in 

 the two species referred to Polycotylus there are four well- 

 formed ones. This may be, in addition to the vertebral char- 

 acters, sufficient to distinguish the genera. 



I give herewith some additional figures of Polycotylus lati- 

 pinnis, made from the Yale specimen 1125 (Plate III, figure 1). 

 The pelvic girdle, as will be seen, is remarkable for the great 

 elongation of the ischia. The paddle figured by me in my 

 previous pajjer on the plesiosaurs* was correctly assigned to 

 the species, but is a hind paddle instead of a pectoral limb. 

 * Field Col. Mus. Pub., Geol. Ser., vol. ii, pi. xx. 



