52 ON THE CAKAPAX AND STERNUM 



represent^s chcz les D6capocles ordinaires que par un seal 

 seo"ment dorsal du k I'ossification diffuse ou fusion des ele- 

 mentes sclerodermique de toute la portion du squelette teg- 

 iimentaire correspondant a ces deux arceaux. Mais chez 

 les Crustaces inferieures, la carapace ne parait avoir d'or- 

 dinaire une composition plus simple, et etre form^etantot 

 par les analogues de I'areeau cephalique seulement, tantot 

 par les representants de I'areeau scapulaire. Ainsi, chez 

 les Squilles, la portion cephalique de la carapace est tres- 

 devellopee ; mais toute la portion posterieure au scapulaire 

 parS,it manquer completcment, et chez les Limnadies, au 

 contraire, I'espece de coquille bivalve, qui tient lieu d'une 

 carapace ordinaire, me parait etre due au developpement 

 excessif de la portion scapulaire seulement, et dependre 

 de I'aimeau mandibulaire, out peut-etre meme de I'un des 

 zoonites suivant. " Owen (3) reflects Milne-Edwards' 

 views throughout as quoted above. Dana (4) differs from 

 Milne-Edwards in that he considers the lateral (ventral) 

 plates of the carapax of crabs to be true /erga instead of 

 epimera {he. cit., p. 27). He infers "that the epistome 

 (or its anterior part) belongs to the second, or to the sec- 

 ond and first normal segments, that is, to the antennulary 

 or to the antennulary and ophthalmic segments. For con- 

 venience of reference I have compiled the following table 

 from the author's statements of his views regarding the 

 number of segments and what parts of each enter into the 

 composition of the crab carapax, 



1. Ophthalmic somite. Parts entirely wanting; appendages, how- 

 ever, present. 



2. Antennulary somite. Sternum present (probal)ly fused with the 

 ophthalmic sternum into one piece) ; the other parts wanting; appen- 

 dages present. 



3. Antennary somite. The parts (sternum, tergum. epislernal plate) 

 present. 



4. Manilibulary somite. The sternum, epislernal plates, epimcral 

 plates and tergum present. 



