HOMOLOGIES OF CRUSTACEA. 29 



The posterior portion of the carapax consists of a postero-lateral re- 

 gion, and a posterior region. 



In the figure here given, the areolets of the frontal region are 

 marked F ; of the orbital, ; of the medial, M ; of the posterior, P ; 



those of the antero-lateral, L ; and those of the postero-lateral, R ; R 

 being the initial of the last syllable of the word lateral, while L is the 

 initial of the first. 



The normal areolets of these several regions are as follows : 



a. Frontal Region. 1 F, the front margin ; 2 F, a prominence just 

 posterior to the front, either side of the middle. 



b. Medial Region. 1 M, two small anterior prominences, the pra)- 

 medial; 2 M, two large areolets, the extra-medial, or gastric of Des- 

 marest ; 3 M, a large central arcolet elongated anteriorly between the 

 areolets 2 M, the intra-medial, or genital of Desmarest ; 4 M, a transverse 

 areolet, just posterior to 3 M, the post-medial. Two deep punctures 

 (over processes on the inner surface that serve for the attachment of 

 mandibular muscles), usually mark the limit between 3 M and 4 M, 

 even when there is no depression. 1 M is here annexed to the 

 medial, rather than frontal region, because it often coalesces with 

 the former, and is a part of it, at times, in general outline. The 

 whole medial region may also be called the gastric. 



c. Antero-Lateral Region. In this region there are normally six 

 areolets : 1 L, near the first tooth following the post^orbital ; 2 L, 3 L, 

 posterior to 1 L, in a line nearly with the second tooth ; 4 L, 5 L, 6 L, 

 between 4 M and the third tooth. 



d. Postero-Lateral Region. This region on either side consists nor- 

 mally of three areolets, 1 R, 2 R, 3 R. 



e. Pbsterior Region. 1 P is situated directly behind 4 M, and is 

 sometimes well circumscribed, and occasionally has the shape nearly 



