236 S. W. WUliston — North American Plesiosaurs. 



the propodials, the great transverse dilatation of the epipodials. 

 and the markedly greater concavity of the posterior border of 

 the propodials (Plate III, figure 2). The metapodials and 

 phalanges are also notable for their shortness and robustness. 

 There are four facets on the distal extremity of the propodials 

 for articulation with the epipodials. 



Length of femur (No. 164-2) 311 n,m 



Greatest distal expanse ._ 154 



Length of humerus (No. 164G) 317 



Greatest distal expanse 172 



An additional species of this genus is known to me, and will 

 be figured and described in a later communication. 



Trinacrohnerum Cragin. 



The genus Trinacrome'rum, if it be distinct from Polycoty- 

 lus, as I believe that it is, is represented in the Yale col- 

 lection by a considerable portion of a skeleton (No. 1129), 

 clearly identifiable with T. anonymum Williston, from the 

 Benton Cretaceous. This specimen, which offers some addi- 

 tional facts of interest, will be figured and described in a later 

 communication, in connection with the description of the type 

 species of the genus, T. bentonianum, which I have recently 

 studied in the Colorado College collection. 



EXPLANATION OF PLATES. 



Plate I. 



Pelvic girdle of Elasmosaurus ischiadicus Williston. p, pubis ; il, ilium ; 

 is, ischium. Specimen No. 1130, Yale Museum. 



Plate II. 



Figure 1. — Eight pelvic paddle of Elasmosaurus ischiadicus Williston. 

 Specimen No. 1130, Yale Museum. 



Figure 2. — Left pectoral paddle of Elasmosaurus (?) marshii Williston. 

 Specimen No. 1645, Yale Museum. 



Plate III. 



Figure 1. — Left half of pelvic girdle of Polycotylus latipinnis Cope, 

 la, pubis; lb, ischium ; lc. ilium. Specimen No. 1125, Yale Museum. 



Figure 2. — Eight propodial and epipodial bones of Polycotylus dolichopus 

 Williston. Specimen No. 1642. Yale Museum. 



Figure 3. — Eight humerus of Elasmosaurus snowii Williston. Specimen 

 No. 1644, Yale Museum. 



Plate IV. 



Elasmosaurus nobilis Williston. Specimen No. 1640, Yale Museum. 



Figure 1. — Eight ilium. 

 Figure 2. — Left ilium. 

 Figure 3. — Eight femur. 

 Figure 4. — Posterior sacral vertebra. 

 Figure 5.— First sacral vertebra. 

 Figure 6. — Anterior dorsal vertebra. 

 Figures 7, 8. — Posterior dorsal vertebrae. 

 Figure 9.— Middle sacral vertebra. 



