REPTILIA. 73 



ard, and the occipital condyles have a similar form. The only teeth are two 

 pointed tnsks growing downwards from the upper jaw ; the lower jaw was arm- 

 ed, like the Tortoise, with a sheath of horn. This specimen, found near Fort 

 Beaufort, Cape Colony, is in the Museum of the Geological Society of London. 



Size, 6x4. Price, $2.25. 



Order 2 — Chelonia. 



These Reptiles, according to Agassiz, are the highest members of 

 this Class, — approaching in some points of internal organization the 

 lower families of aquatic Birds. The main ordinal characteristic is the 

 solid, immovable armor encasing the greater part of the body. The 

 dorsal shield or carapax is least complete in marine Turtles, as the ribs 

 extend beyond the costal plates, leaving unossified intervals. This fact 

 affords an important aid in the discrimination of fossil Chelonians. In 

 all, excepting the soft mud Tortoises, the whole armor is covered with 

 dermal plates or ossified skin — homologues to the scutes of the Crocodile. 

 The Order is also distinguished by a toothless horny beak and a single 

 external nostril placed in the middle of the forepart of the skull. The 

 lower jaw is one solid arch. The only movable vertebras are the cervical 

 and caudal. Some of the cervicals are convexo-concave, others concavo- 

 convex, one biconcave (usually the 8th). The caudals are procoelian. The 

 sternum is broad in the land species, narrow in the marine. The land 

 species furnish the first instance of real walking in the Vertebrate 

 series, unless the running of some Toads be considered as such j for 

 Salamanders, Lizards and Crocodiles depend partly on the wriggling of 

 the spinal column. The feet of the marine Chelonians are fin-shaped ; 

 of the fluviatile and marsh species, palmated ; of the land species, 

 club-shaped. 



The two shields, usually in fragments, are the chief evidence of 

 extinct Chelonians. The beaks are sometimes found solitary in the 

 Chalk. According to Agassiz, the first genuine Testudinata belong to 

 the Oolitic series. The so-called Chelonian footprints on the Potsdam, 

 Old Red, and Triassic Sandstones are very uncertain. The earliest 

 species are Emydians from the Solenhofen quarries. 



