ORDER CHELONIA. III3 



gerhead forbids this view. We have already suggested reasons for 

 regarding this family as being descended from the Mesozoic Aci- 

 chelyidce, and from this point of view it is interesting to note the 

 presence of an intergular shield in the plastron of the existing, and 

 probably, therefore, of the fossil forms, since, as we have already 

 stated, this appears to be an archaic feature. The same observa- 

 tion will apply to the open tympanic ring. At the present day 

 there are four living species of Turtles — viz., the Loggerhead 

 (Thalassochelys caretta, fig. 1006), the Mexican Loggerhead (T. 

 Kempi), the Hawksbill (Chelone imbricata, fig. 10 18), and the 

 Green Turtle (C. my das), all of which are of purely marine habits. 

 The Hawksbill alone is carnivorous j and is further peculiar for the 

 circumstance that in the young state the epidermal shields imbricate, 

 instead of uniting by their edges. It has been suggested that the 

 extinct genera were of estuarine rather than purely marine habits ; 

 and this is borne out by their occurrence in the estuarine deposits 

 of the London Clay, to the exclusion of the marine genus 

 Chelone. 



The most remarkable of the extinct genera is Lytoloma (Eudastes, 

 Puppigerus, Glossochelys, or Erquelinnesid), which occurs typically 

 in the Eocene of North America and the London Clay, and is also 

 represented in the Middle Eocene of Bracklesham, as well as in the 

 Chalk and Cambridge Greensand. In the typical Eocene forms, 

 like the so-called Chelone plani?nentiim and C. crassicostatum of the 

 London Clay, the skull is as large in proportion to the shell, as in 

 Macrocle7nmys among the Chelydridce. The skull of the adult is 

 remarkable for the extremely backward position of the posterior 

 nares, which are approximated to the basioccipital ; and for the 

 length and width of the mandibular symphysis (fig. 1019, a). The 

 palate has low alveolar walls, and no oral ridge ; the nares and 

 orbits are directed somewhat upwardly ; and the bar between the 

 latter is narrow. The shell is characterised by the great extent of 

 its ossification, as well as by the rounded posterior extremity of the 

 carapace, and the sutural connection of the broad xiphiplastrals in 

 the median line. The head of the humerus is somewhat oblique, 

 and its shaft much constricted. The limbs were clawed. Curiously 

 enough, in the young, and perhaps also in the adult of some of the 

 smaller forms, the posterior nares were placed much less back- 

 wardly, and the mandibular symphysis was shorter and less 

 flattened. It is probable that at least some of the North American 

 Eocene forms described as Osteopygis, Propleura, and Catapleura, 

 are not separable from this genus ; while a turtle from the Creta- 

 ceous of Australia, originally described by the preoccupied name 

 of Notochelys, but which has been provisionally designated Noto- 

 chelone, is probably also nearly related. 



