32G AMERICAN TESTUDINATA. Part IL 



here both carapace and plastron composed chiefly of bony plates resting immediately 

 upon, and firmly fixed to the true skeleton, and united to one anotber. The only 

 part of the carapace which remains unossifled up to adult age is a narrow strip 

 along the ribs near their lower ends, just above the ossified marginal rim, and 

 extending all round except at each end, where a bony plate is interposed. All 

 the ribs, except the first and tenth, reach down to the marginal rim. The eight 

 other ribs have each a bony plate extending from the inner end outward ; but 

 these bony plates do not reach the bony marginal rim, or if at all, not till late in 

 life. The first rib rests on the same plate with the second, and so also the tenth 

 with the ninth. Between the inner ends of each pair of costal plates, above the 

 vertebral column, and firmly fixed to it, there is a small plate filling the whole 

 space ; the number of these plates varies somewhat, as one or more of the hinder 

 ones is often divided. In front, an odd plate extends from tlie foremost j^late of 

 the vertebral row, and from between the foremost pair of costals to the front end 

 of the carapace, thus entering into the marginal rim, and connecting it with the 

 bony derm above. This plate does not touch immediately any rib or vertebra, but 

 is connected Avith the isolated true bone situated above the lower neck vertebraB, 

 and the connection is so intimate that they can hardl}'' be distinguished apart. 

 The ninth pair of ribs reaches almost directly backward, passing over the iliac bones, 

 and giving support to the narrow, pointed hind end of the body. Wedged between 

 the plates wliich are fixed to these ribs, and behind the last of the plates which 

 are fixed to the vertebrca, there is one 13'iug over the sacrum, but free from it ; 

 sutured to this there is another behind, and sutured to the Litter still another, 

 which last enters into the marginal rim and terim)iates it behind. The jilates of 

 the marginal rim are in one continuous row all round, consisting generally of 

 eleven pairs ^ besides the odd one at each end ; two of these pairs are in advance 

 of the first costals. The costal plates are firmly fixed to the ribs and sutured to 

 one another, and those of the vertebral row are firndy united to one another and 

 to the costals, and those which are fixed to the A^ertebraj are firmly soldered to 

 them ; the marghial plates, passing along the ends of the ribs, connect them with 

 one another, and they are themselves connected with the bony derm above by 

 the odd plates at the ends of the carapace. Thus we have a combination of 

 bony derm with the vertebra? and ril)S wdiich is well adapted to give strength 

 and stability to the broad, roof-Uke carapace. 



The plastron is connected with tlie carapace at the lower edge of the mar- 

 ginal rim by unossified corium, and is somewhat movable or rather yielding there, 

 as it also is along its middle line for the greater part of its length. In Sphar- 



^ The scales which cover these plates are not so constant. 



