XIV TNTRODUCTION. 



incisive margin, the two extremities or horns of a cres- 

 cent. The second, or articulated process, is situated 

 between the two, but somewhat nearer the anterior 

 margin. It appears to be able to assist in carrying the 

 food from the one point to the other, from the biting to 

 the grinding surfaces, between which and the molar 

 tubercle are frequently a row of strong and curved spines 

 that facilitate the process. 



The mandibles are moved by powerful muscles attached 

 to the inner surface of the dorsal part of the cephalon, 

 corresponding with the homological parts that are 

 attached to the inner dorsal surface of the carapace 

 of the higher Crustacea. 



The surface of the molar tubercle is granulated with 

 rows of minute denticles that are only visible under a 

 strong magnifying power. In some species, a long and 

 slender ciliated filament is appended to the margin of the 

 tubercle that may be associated with the sense of taste. 



The mandibles are no exception to the fact that all 

 appendages are but modified legs. In all Crustacea, we 

 think that it can readily be demonstrated that the man- 

 dible consists of the first three joints being closely anchy- 

 losed. The small appendage, that generally consists of 

 three freely articulated joints, represents the fourth, fifth, 

 and sixth joints; the seventh, or dactylos, being seldom 

 present. An homological examination of the genera 

 Nebalia and Pontia, with Homarus, together with the 

 homotypical parts in other appendages in the same ani- 

 mals, we think will readily confirm this opinion. The 

 small three-jointed appendage to the mandible is wanting 

 in but few genera, excepting in the terrestrial Isopoda and 

 AmpMpoda. In aquatic species it is, with few exceptions, 

 always present, and appears to be of efficient use in 

 directing floatino; material towards the mouth. 



