26 CRUSTACEA. 



portion of the carapax. The suture is so marked, that in many species, 

 even when fresh, the carapax breaks readily along its line ; and gene- 

 rally, the suture opens very easily when the carapax has been 

 weathered by exposure, if not before. Near the anterior angle of this 

 ventral piece there is an oblique suture often apparent, very near the 

 angle of the buccal area (see Plate 11, fig. 9d, f, and Plate 13, fig. Qh, 

 also Plate 16, fig. 9 c), extending backward to the margin of this area; 

 and it separates a small part of the ventral piece, which piece seems 

 generally to be continued some distance backward, as a narrow mar- 

 gin to the ventral piece. 



It becomes a question of interest, what is the proper relation of the 

 ventral pieces of the carapax ? Are they true epimerals or not ? 

 There is certainly a difficulty in the way of admitting them to be epi- 

 merals. We have pointed out the sternal and episternal pieces of the 

 prselabial piece. Beyond the episternals, the epimerals normally come 

 next in order. But the next piece is, in fact, the small plate, / (fig. 

 9, Plate 11), and then comes the larger one behind; so that, if/ is the 

 epimeral, as it should be, the latter is not so. This difficulty is 

 avoided only by supposing the suture separating / from the part 

 behind, to be unessential, and the whole, therefore, to be properly one 

 piece, or the true epimeral. 



But there is an additional difficulty which, in connexion with other 

 facts, throws doubt upon this received opinion. Although each ventral 

 piece, or rather the part/, actually adjoins the anterior portion of the 

 prselabial plate, the suture p* is very distinct, while the outer and pos- 

 terior portion of the prselabial plate passes into the piece / with perfect 

 continuity, and with often a solid, shelly texture throughout. This con- 

 tinuity proves a closer relation with the posterior and outer portion of 

 the praslabial plate, than with the episternal pieces themselves. It 

 shows that the posterior portion and the ventral pieces are one united 

 plate ; and if this posterior portion of the praelabial plate belongs to the 

 mandibular segment, then the piece /, which so solidly coalesces with 

 it either side, is also mandibular in its relations. Indeed it seems 

 altogether probable that this is the true view of the subject. The 

 piece /is the epimeral of the mandibular segment, and g is the follow- 

 ing or dorsal portion of this segment. On this ground we understand 

 the ready disjunction of the carapax and ventral pieces; for they are 

 actually distinct segments of the body. The forward extension of the 

 piece / alongside of the episternals of the anterior margin of the prge- 



