206 VERTEBRATES. 



4. A terminal caudal region, of many small vertebrae, 

 without developed ribs, tapering posteriorly to the tip. 



The dorsal vertebrae are the ones concerned in the 

 formation of the carapace. Observe that, of the ten dorsal 

 vertebrae, each one, except the first and the last, bears a 

 neural plate and a pair of costal plates. Observe that 

 the ribs from the first dorsal attach to the costal plates of 

 the second, and that the ribs from the tenth attach to the 

 costal plates of the ninth. Notice that several of the 

 anterior pairs of ribs appear to have moved forward until 

 they articulate with the centra of two vertebrae. Notice 

 the finely toothed sutures by which the costal plates meet 

 each other. 



How many of the points noted in the structure of the 

 shell are directly conducive to its strength? 



How many caudal vertebrae are there? How many 

 cervical? Observe that the first cervical meets the skull 

 by a single occipital condyle. Separate the cervical ver- 

 tebrae, and note the great variety of articulating surfaces 

 between the ends of adjacent centra. 



IV. Girdles and Leg Bones. In each half of the 

 shoulder girdle there are three bones : 



1. The bone which extends from the shoulder dorsally, 

 to meet the first costal plate, is the scapula. 



2. The posterior of the two bones which extend from 

 the shoulder toward the median line is the coracoid. It 

 is horizontally flattened toward its inner end, there be- 

 coming somewhat triangular. 



3. The anterior of the horizontal bones is the pre- 

 coracoid. 



In each half of the pelvic girdle there are three 

 bones : 



1. The bone which extends from the acetabulum dor- 

 sally, to meet the sacral ribs, is the ilium. 



2. The posterior of the two bones which extend from 



