30 CRUSTACEA. 



of 3 M reversed and shortened; it is the cardiac of Desmarest, and may 

 be so called. 2 P ; directly behind 1 P, may be either simple, or, as is 

 more common, divided into two areolets; Desmarest's designation, the 

 intestinal, is appropriate. 



/. Orbital Region. The elevation which forms the upper side of 

 the orbit is usually divided by sutures into three parts. 



The variations in the markings of Crabs arise in the main from the 

 greater or less prominence of these areolets, their various subdivisions 

 or their obsolescence. When there are only a few undulations on the 

 carapax, a little study with the normal type in view, will commonly 

 discover that the system is there, although it may be but just apparent. 



In the obsolescence of the areolets, the posterior are the first to dis- 

 appear, and when so, this part of the surface is flat or only undulated. 

 Next the postero-lateral fail ; next, 5 L and 6 L coalesce, and also 1 M 

 and 2 M. Next the posterior of the medial areolets become obsolete, 

 and at the same time 5 L, 6 L disappear, or are indicated only by a 

 slight undulation along the space that ordinarily separates them. The 

 extra-medial may be circumscribed only anteriorly, and the slender 

 elongation of the intra-medial be all that appears of that areolet; 

 next, the remaining antero-lateral areolets may disappear with the 

 frontal, and the surface is then quite smooth. 1 L is sometimes indis- 

 tinct when the others are prominent, though usually it accompanies 

 them. 



When 4 L, 5 L, 6 L become indistinct, the transverse depression, 

 described as separating the posterior and anterior portions of the cara- 

 pax, may be obsolete, and the transverse line bends more forward, 

 passing along by the side of 3 M, then anterior to 5 L, but not anterior 

 to 4 L, in which case, the transverse line has nearly a straight, trans- 

 verse course. Again, the line may pass anterior to 5 L, to the interval 

 between the teeth N and T, or between E and N ; and in the latter 

 case it is often deep, as in the Oxyrhyncha. 



In the subdivision of the areolets, the first that partake of it are 2 M, 

 5 L, and 3 M. A commencement of this division of 2 M, the extra-me- 

 dial, is very common, and when completed, it divides it into two parts 

 longitudinally.* 5 L also subdivides from above across obliquely and 

 inward; 3 M subdivides at times into three parts, as shown in some of 

 the Chlorodii (see Atlas), and occasionally is farther divided. Again, 



* This is an important specific character, and though hitherto unmentioned in descrip- 

 tions, is easily described when a proper notation is adopted. 



