lO ANATOMY OF A CHELOMIAN. 



On its under surface is the cloaca! opening ; a trans- 

 verse slit, with puckered edges. 



7. The Epidermic Exoskeleton mainly consists of homy 

 plates (tortoise-shell), of definite number and very con- 

 stant arrangement, found covering carapace and plas- 

 tron. 



a. On the plastron are sixteen pieces, twelve ar- 

 ranged in pairs on the sides of the middle line, 

 and the remaining four covering the margins 

 of the bony bridges uniting carapace and plas- 

 tron. 



b. On the carapace, in the middle line in front, is 

 a small nuchal plate ; and, meeting in the mid- 

 dle line behind, over the base of the tail, are a 

 pair oi pygal plates. Running along the middle 

 of the back, from nuchal to pygal plates, are five 

 unpaired central plates. On each side of the 

 latter are four centro-lateral plates; and, external 

 to these, eleven marginal plates on each side, ex- 

 tending along the edge of the carapace from 

 the nuchal to the pygal plates, and, like them, 

 curving around the edge so as to cover in the 

 projecting margin of the carapace on its ventral 

 side. 



c. The claws, the horny coverings of the jaws, and 

 the scales (which are especially developed on 

 the paddles and the dorsal side of the tail) are 

 also parts of the epidermic exoskeleton. 



8. Make drawings showing the arrangement and form 

 of the epidermic plates on carapace and plastron. Then 

 immerse the animal in boiling water for about two min- 



