HOMOLOGIES OF CRUSTACEA. 3} 



2 M is not only divided in halves longitudinally, but each part is again 

 divided. In the Daira perlata,* the outer half of 2 M consists of 

 four tubercles, and the inner of three ; 1 M consists of one ; and 3 M 

 is also divided into several tubercles. In some species, a portion is 

 separated from the anterior part of each half of 2 M, while the rest 

 remains entire. 



It is common also for 1 B, to be subdivided, or to have one or two 

 tubercles upon its surface adjoining the transverse depression S S. 



g. Teeth of ilie Antero-lateral Margin. — The teeth of the margin are 

 normally five in number, including the post-orbital as the first. These 

 five, in the preceding figure, are designated by the different letters of 

 the Latin word denies (or French, dents), D, E, N, T, S. Each tooth 

 is often separated from the preceding by a suture, and these sutures 

 may be continued on the under surface. The letters hence mark 

 properly a lobe of the margin, rather than simply a tooth. 



The teeth vary by obsolescence or subdivision, like the areolets. 



In obsolescence, the tooth E (second), is commonly the first to dis- 

 appear, this reducing the apparent number to four. Then N fades 

 out, then T, leaving S alone, which also may be wanting. Again, S 

 is sometimes smaller than T, or disappears altogether; in many species 

 N, T, S, are all wanting. 



In the multiplication of teeth, there is often, as a first addition, a 

 tooth a* (or two s' ', s") posterior to S; it corresponds to the fold in the 

 under surface shown at s' in figure 9 d, Plate 11. 



There is often also a tooth d', between D and E, on a lower level 

 than D. 



But the multiplication is generally dependent on the subdivision of 

 the normal teeth, E, N, T, in addition sometimes to S and D; each of 

 these teeth consisting of two or three teeth, either all equal, or one 

 more prominent. In order to determine the normal relations of the 

 teeth when the number is large, there is a guide in the areolets ad- 

 joining, when they exist ; for the areolet 4 L stands against tooth T, 

 and may be viewed as pertaining to the same lobe, it having about 

 the same breadth as belongs to this lobe. So 3 L (or 3 L, 2 L), gives 

 the breadth of the normal tooth or lobe N; and 1L when present 

 that of E. The lobes, or teeth, are often a little posterior to the 

 areolets, or are in the same transverse line instead of anterior to them. 



* See Plate 10, fig. 4, where the separate regions are distinguished. 



