CHELYDRTD.U. 133 



matemys ; there are, however, five costal shields, and the 

 fourth vertebral shield is in consequence proportionately 

 elongated. The marginals on the right side are fairly 

 preserved, and show a distinct line of separation from the 

 costals ; in the posterior costals of this side the articular 

 extremity of the rib is well shown. 



Presented by Sir R. Owen, K.C.B., 1880. 



Family CHELYDBIDiE. 



Shell usually not fully ossified till a late period ; plastron fre- 

 quently small, with or without vacuities, and articulating with the 

 carapace by gomphosis, without axillary and inguinal buttresses ; 

 nuchal emarginate, with a long costiform process underlying the 

 anterior marginals. In existing forms most of the caudal vertebrae 

 opisthoccelous ; chevrons present. Skull (when known) more or less 

 triangular in form and wide posteriorly, with the temporal fossae 

 partially roofed, the squamosal not being connected with the 

 parietal ; no floor to narial passage ; tympanic ring of typical forms 

 in great part closed posteriorly. Scapula joining precoracoid at a 

 right angle. Humerus (fig. 31), when known, with the well- 

 developed and broad head placed obliquely to the curved shaft, and 

 the radial process forming a strong ridge at right angles to the 

 dorsal surface of the shaft, and the ulnar process also deflected ven- 

 trally. Limbs not modified into paddles. Pelvis with an obturator 

 notch. Epidermal shields absent in some cases. Skull relatively 

 large, and the head incompletely retractile. 



Our imperfect knowledge of the fossil forms provisionally included 

 in this family renders it necessary to regard the foregoing diagnosis 

 as purely provisional, more especially as there are indications 

 among fossil forms of a more or less complete transition between 

 the present and preceding families. 



Subfamily Chelydrin^e. 



Shell smooth and covered with epidermal shields ; the plastron 

 small and cruciform, with vacuities persisting till a late period, a 

 narrow bridge, and an oblique suture between hypo- and xiphi- 

 plastrals ; 11 marginals ; neurals of the full number, and short and 

 wide ; caudal vertebrae mostly opisthoccelous. Vertebral shields 

 wider than long. 



