HOMOLOGIES OF CRUSTACEA. 



labial plate, produces the suture alluded to. The carapax in this case 

 is mainly the second antennary segment. It unites with the epister- 

 nals of the same second antennary segment just at the base of the 

 second antennae. The suture about the plate f g, anteriorly is some- 

 times so open, that on this ground alone, we should infer its belonging 

 to a distinct segment ; and moreover it sometimes appears partly to 

 overlie at the margin the adjoining parts, showing still farther that 

 it is probably a separate segment. 



If these deductions are correct, the epimerals of the second anten- 

 nary segment or carapax are not distinguishable, even in outline. A 

 fissure or emargination in the under side of the orbit, common in the 

 Brachyura, may perhaps indicate its limit, or perhaps, some of the 

 sutures or depressed lines on the back of the carapax; but this is only 

 conjecture. No objection to our view can thence be urged, since 

 there is no special reason for expecting that the epimeral should be 

 distinct, and much less for inferring that it should be as separable by 

 fracture, as is the case between the carapax and its ventral pieces. 

 We are therefore led to believe, that the so-called epimerals, or ventral 

 pieces of the carapax, are in fact the posterior extensions of the mandi- 

 bular segment. 



In the preceding explanations, we have intended to draw a line 

 between the epistome, or rather its anterior portion, and the front 

 margin of the praelabial plate. In many species the two are united 

 in an unbroken surface; but in others, there is a deep and open 

 suture, and in some, as the Chlorodius, referred to above (Plate 11, 

 fig. 9d), there is an unossified membrane (between e and e' of the 

 figure referred to). We have shown that the second antennae are 

 connected with the praelabial plate, rather than the epistome, when 

 the distinction can be drawn. But this epistome is immediately con- 

 nected, anteriorly, with the inner antennae, and is continued within 

 either side, so as to form the inner walls of the orbit. We have rea- 

 son, therefore, for inferring that the epistome (or its anterior part) 

 belongs to the second, or to the second and first normal segments — that is, 

 to the first antennary, or the first antennary and the ophthalmic segwt nts. 

 The latter is the more probable view ; for, otherwise, the ophthalmic 

 segment must be obsolete, while the eyes that pertain to it are 

 present. The anterior extremity of the epistome, or the inter-auten- 

 nary septum, is sometimes prolonged into a spine, as in some Oxy- 

 rhyncha, which is more prominent than the front of the carapax; but 



