52 ON THE CARAPAX AND STERNUM 



represented chez les D6capodes ordinaires que par uu seul 

 segment dorsal dti. a 1'ossification diffuse on fusion des ele- 

 mentes scleroderrnique de toute la portion du squelette t6g- 

 umentaire correspondant a ces deux arceaux. Mais chez 

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

 dinaire une composition plus simple, et etre formee tantot 

 par )es analogues de 1'arceau cephalique seulement, tantot 

 par les representants de 1'arceau scapulaire. Ainsi, chez 

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

 devellopee ; mais toute la portion posterieure a*i scapulaire 

 parait manquer completement, et chez les Limnadies, au 

 contraire, 1'espece de coquille bivalve, qui tient lieu d'une 

 carapace ordinaire, me parait tre due au developpement 

 excessif de la portion scapulaire seulement, et depeudre 

 de I'anueau mandibulaire, ont peut-6tre meme de 1'un des 

 zoonites suivaut. " 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 ot crabs to be true terga instead of 

 epimera (loc. 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 (probably fused with the 

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

 dages present. 



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

 present. 



4. Mandibulary somite. The sternum, episternal plates, epimeral 

 plates and tergum present. 



