24 CRUSTACEA. 



mity, between the mouth and the inner antennae, is the lower arch 

 complete, by a junction of the sides across the ventral surface. This 

 anterior portion is then the true ring, and the posterior part is only a 

 backward expansion of it; and the carapax must therefore pertain to 

 the same segments which constitute the lower arch. This lower arch, 

 or inferior surface (Plate 11, fig. 9rf), is the praelabial space (p), 

 and epistome (e e') ; posteriorly it is articulated with the mandibles, 

 and anteriorly with the second antennas (a 2 ), whence its normal rela- 

 tions lie between the mandibular and second antennary segments, one 

 or the other, or both. 



The second antennae in some cases seem to be articulated as much 

 with the epistome inside of the basal portion of the basal joint as with 

 the part posterior to it. But in other species, its actual, intimate 

 connexion with the anterior margin of the praelabial area, is evident. 

 It is often prolonged backward, much beyond the part of the epistome 

 adjoining it on the inside ; and it is frequently soldered to the prae- 

 labial plate, so as to be continuous with it, while an open suture sepa- 

 rates it from the epistome, a fact indicating its closer connexion 'with 

 the praalabial plate. Again, as in a Lithodes, its direct articulation 

 with the margin of the prtelabial plate is distinctly obvious. There 

 seems, therefore, to be no doubt that the anterior portion of the prje- 

 labial plate pertains to the same segment as the second antennae. 



In order to arrive at an answer to the question, which of the two 

 segments, the second antennary or the mandibular, corresponds to the 

 carapax, we have examined with some care the sutures in the prae- 

 labial plate, and those of the carapax, either side of the buccal area. 

 The suggestions thus obtained are of considerable interest. 



The praelabial plate, as is well known, has generally three emargi- 

 nations in its anterior margin (p,p l ,p*, Plate 11, fig. 9rf, Chlorodius 

 monticulosus), and these emarginations are the terminations of sutures, 

 which usually are readily distinguished on the surface of the plate. 

 The median suture (p) extends back more than half-way to the pos- 

 terior margin of the plate, and is often more open where it terminates. 

 The next, either side (p 1 ) continues backward a short distance, and 

 then curves inward; the outer (p 2 } takes nearly the same course, and 

 leaves an outer and obliquely posterior portion of the plate outside of 

 it. The pieces between these sutures appear to correspond to the two 

 sternal plates between p and p 1 , either side ; and to the episternal be- 

 tween p 1 and p\ Now, it is the episternal, with which each of the 



