SKELETON 



59 



persistent notochords, others amphicoelous centra. In the plesiosaurs they 

 were flat, whUe in the turtles the dorsals are fused and the neural spines are 

 united with the neural plates (p. 4S). The other centra vary. Those of the 

 rhynchocephals and most dinosaurs are flat, whUe snakes and lizards (except the 

 geckos) have them procoelous. In the earliest crocodiles they were amphicoelous, 

 whUe later they are procoelous or flat, and in the pterodactyls they are procoelous 

 in front, amphicoelous in the tail. 



BIRDS usually have saddle-shaped 

 ends to the centra (the atlas procoelous). 

 Several of the dorsals are usually fused for 

 strength, but the first presacral is free. A 

 characteristic feature is the synsacrum 

 (fig. 56), foreshadowed in the dinosaurs. 

 As the bird stands on two feet and holds 

 the body obliquely, several of the dorsal 

 and caudal vertebrae (up to 20) have fused 

 with the true sacrals into a common 

 synsacral mass, a large proportion also 

 uniting with the pelvis, thus give a /RSI 15^® \v^P 



Fig. 55. Fig. 56. 



IiG. 55. — Cervical vertebra of a bird showing the saddle-shaped articular surface 

 (a/) on the centrum, c; cr, cervical rib; nc, neural canal; W5, neural spine; poz,prz, post- 

 and prezygapophyses. 



Fig. 56. — Ventral view of synsacrum and pelvis of hawk {Buteo). il, ilium; is, 

 ischium; p, pubis; pp, pectineal process; s, sacral ribs. 



firmer framework for the attachment of the muscles of the legs. The 

 true sacrals (three in ostriches, two elsewhere) lie just behind the pits occupied 

 by the kidneys and may be recognized by their lower articulation with the pelvis. 

 A few of the caudals behind the synsacrum are free, as all were in Archoeopleryx, 

 but the others in recent birds are united into an upturned bone, the pygostyle. 

 MAMMALS, except whales where the sacrum is lacking, have all five verte- 

 ral regions differentiated. With four exceptions the cervicals are seven in num- 

 tr {Manatus australis and Cholcepus hofmanni, six; Brady pus iorquatus, eight; 

 B. tridactylus, nine). The dorsals (thoracics plus lumbars) vary between four- 

 teen in armadillos and thirty in Uyrax, but usually are nineteen or twenty; in- 

 crease in the number of thoracics usually being at the expense of the Jumbars. 

 There are primitively two sacrals, but others may unite with these until they 

 mount to nine or ten in some edentates. Usually the centra are amphiplatyan, 

 but in the cervicals of ungulates opisthocoele vertebrae are common. It is to be 



