34 



Ai^ATOMY 05 VERTEBRATES. 



Lfpidostcus 



as to obliterate the front cavity, and protrude into the liind cavity 

 of the preceding vertebra, fig. 28 ; thus establishing a cup-and- 

 ball articulation on the ' opisthocoelian ' plan. 

 The cup-and-ball structure prevails throughout 

 the air-breatliing, land-seeking, or terrestrial, 

 Hamatocrya. So interlocked, the vertebra are 

 better fitted to support the body in air, and 

 transfer its weight to legs. Sometmies the cup 

 is behind, as in the land-salamander, the Surinam 

 marm, ^Q^d (Pipa), and some extinct crocodiles, thence 



called Streptospondylus ; but, as a general rule, existing reptiles 

 have ' procoelian ' vertebras, or with the cup in front. In many 

 extinct reptiles {Sauropterygia, Dinosauria) ossi- 

 fication was so advanced as to leave no cavity 

 at either end of the centrum ; and these parts 

 were coarticulated by flattened or almost flat- 

 tened surfaces, as in mammals. Finally, both 

 extinct and recent Keptilia afibrd instances in 

 which the parts or elements of the vertebra have 

 coalesced into one bone. 



The progressive stages in the developement of 

 a vcrteljra, which have been illustrated by the chief of those at 

 which it is arrested in the cold-blooded series, bear a close analogy 

 to those by which it reaches the coalesced condition as a siugle 

 bone in the warm-blooded classes. The principal secondary and 

 adaptive modifications will next be pointed out which mark with 

 special characters the collective trunk-vertebras in Hamatocrya. 



§ 17. Vertehrul column of Fishes. — In the Stiu-geon {Aci- 

 penser), fig. 29, the first five or six neural arches are confluent 

 with each other and with the parapophyses, forming a continuous 

 sheath of firm cartilage (fig. 62), inclosing the fore part of the 

 notochord, ib. a, and myelon, and perforated for the exit of 

 the nerves. The tapering end of the notochord is continued 

 forward into the fused basal elements of the cranial vertebras, 

 ib. ff, (f, and backward into the base and upper lobe of the 

 tail-fin, fig. 29, c. The vertebras are represented by their 

 peripheral elements, and ])rincipally by the neural and hasmal 

 arches. The pleurapo])liyses are limited to about twelve of 

 the anterior trunk-vertebras, are articulated by simple heads 

 to parapophyses, fig. 62, p, and rapidly shorten in the two or 

 three hinder pairs ; the large ones sometimes consist of two or 

 three pieces joined end on end, like the modified occipital rib, 

 called ' scapula.' Vegetative repetition of perivertebral parts 



