ORDER VI CHELONIA 207 



may articulate, without sutural union, with the xiphiplastral. The coracoid 

 and humerus are of a Pleurodiran type. 



Family 1. Pleurosternidae. Lydekker. 



A provisional assemblage embracing a variety of generalised fossil forms, 

 and to be regarded as ancestral to both Cryptodires and Pleurodires. 



Pleiirosternum, Owen- {Megasternum, Gray). Carapace much depressed, 

 rounded posteriorly, without vacuities, and firmly united by a long bridge on 

 each side with the plastron, which is also continuous. Surface of shell very 



L.S«^ 



Fig. 808. 



Platychdys oherndorfeii, Wagner. Upper Jura ; Kelheim, Bavaria. 1/4- '^j Carapace. B, Plastron. jS.S, 

 Bracliial shield ; Ep, Epiplastroa ; OS, Gular shield ; IIpp, Hypoplastron ; Hyp, Hyoplastron ; /G,S', tntergular ; 

 LS i-iv, Laterals ; Mp, Mesoplastron ; MS, Marginal ; NS i-v, Neural or vertebral shields ; SmS, Supramargiiial ; 

 Xp, Xiphiplastron ; ci-8, Costal plates , Entoplasti'on «i-8 Neural plates ; nuch. Nuchal ; 2}y, Pygal ; x. Pit 

 for attachment of ilium. ' 



finely pitted and deeply impressed with the suture-lines of the epidermal 

 shields. Neural bones moderately elongated, hexagonal, with their antero- 

 lateral surfaces short. Mesoplastrals well developed, meeting in the middle 

 line ; entoplastral relatively large ; xiphiplastrals deeply notched. No nuchal 

 shield ; intergular shield not divided ; inframarginals present. Pectoral arch 

 resembling that of the existing Chelys. Upper J ura ; England. 



Platychelys, Wagner {Helemys, Riit.), (Fig. 308). Carapace somewhat 

 depressed, with a number of more or less distinct ridges or prominences on the 

 neurals and costals. Neurals short and of irregular contour, the width usually 

 much exceeding the length. Bridge short. Mesoplastrals not meeting in the 

 middle line. Vertebral shields much wider than long ; nuchal shield present ; 

 intergular undivided. No articulation between pelvis and plastron (Lydekker). 

 Upper Jura ; Bavaria, France, and England. [This genus is included by von 

 Zittel with the Chelyclridae.'\ 



Ba'ena, Leidy. Intergular shield divided, and xiphiplastral with depressions 

 which appear to have received the pubis and ischium. Mesoplastrals meeting 

 at the middle line, but with the median ends much narrower than the outer 

 ends. Upper Cretaceous and Eocene ; United States. 



