ON DECAPOD CRUSTACEA. 57 



trace the connection of the two portions of the carapax 

 with comparative ease. The episternals and epimerals of 

 the antennary segment are calcified. The former appear 

 on either side of the epistome or sternum as an oblong 

 plate extending backward, downward and outward and 

 also furnish the upper plate of the entrance to the gill- 

 chamber. The epimeral plate is folded inward close upon 

 the episternum of either side and is only to be seen when 

 the edges of the carapax are spread apart. The episterna 

 and epimera of the mandibular segment are represented 

 by slightly calcified membranes more or less folded upon 

 themselves. These plates are related to the mandibular 

 sternum in a manner similar to that stated for that of the 

 corresponding plates of the preceding segment. In Squilla 

 the antennary sternum is especially prominent and reaches 

 backward and downward in the form of a half cylinder, 

 the sides of which are formed by the large episternal 

 plates. The carapax is almost entirely made up by the 

 antennary tergum, and the antennary somite furnishes fur- 

 ther, fully one-half of the length of the cephalo-thoracic 

 region of the body of this crustacean. The statement that 

 the terga of the thoracic somites are covered by the ceph- 

 alo-thoracic shield is not strictly true. The first two terga 

 (counting from behind forwards) are entire and free, the 

 third is united by a membrane to the posterior edge of the 

 cephalothorax. The remaining terga are incomplete and 

 unite with the carapax in a line curving from the point of 

 attachment of the third, outwards and forwards on either 

 side of the median dorsal line of the body (fig. 21). 



Sternal plates, etc. Milne-Edwards considers the 

 small calcareous plates found at the base of the thoracic 

 appendages, which in the adult state are more or less fused 

 with the sterna of the respective segments, to be the ho- 

 mologues of the episternal pieces of the typical arthropod 



ESSEX INST. BULLKTIN, VOL. XVII. 8 



