carapace （カメなどの）背甲

plastron （カメなどの）腹甲

Turtles, like frogs, cannot be mistaken for any other animal (Fig. 1.3). The body is encased within upper and lower bony shells (carapace and plastron, respectively). In some species, the upper and lower shells fit tightly together, completely protecting the limbs and head. Although turtles are only moderately speciose, they are ecologically diverse, with some fully aquatic (except for egg deposition) and others fully terrestrial. Some are tiny in size whereas others are gigantic, and some are herbivores and others are carnivores.

Laurie J. Vitt and Janalee P. Caldwell, Herpetology : An Introductory Biology of Amphibians and Reptiles, 3rd ed., Academic Press, 2009, p.4.

carapace s. f.
Test osseux qui recouvre le corps des tortues et, en général, des reptiles chéloniens. C’est le bouclier supérieur ou dorsal de ces animaux ; le bouclier inférieur porte le nom de plastron. [Littré]
ÉTYMOLOGIE : Espagn. carapacho. Une calebasse se dit en catalan carabassa, en sicilien caravazza ; il n’y a pas loin pour passer de là, quant à la forme et quant au sens, à carapace.

carapace n technical
a hard shell on the outside of some animals such as a crab or tortoise [LDCE]

carapace n
the thick hard shield, made of chitin or bone, that covers part of the body of crabs, lobsters, tortoises, etc [CED]

carapace n SPECIALIZED
a hard shell that covers and protects animals such as crabs and turtles [CALD]

carapace n.
Zoology. A hard bony or chitinous outer covering, such as the fused dorsal plates of a turtle or the portion of the exoskeleton covering the head and thorax of a crustacean. [AHD4]

carapace n.
1 : a bony or chitinous case or shield covering the back or part of the back of an animal (as a turtle or crab) [MWCD]

plastron n
the bony plate forming the ventral part of the shell of a tortoise or turtle [CED]

plastron n.
Zoology. The ventral part of the shell of a turtle or tortoise. [AHD4]