212 CLASS CRUSTACEA. 



Typhis, Risso, 



Have but two very small antennae. The head is thick, and 

 the eyes not projecting. Each pair of feet is annexed to a 

 proper segment : the anterior four are terminated by a didac- 

 tylous pincer. On each side of the thorax are two mobile 

 laminae, forming sorts of shutters, or valves, which, united, 

 when the animal folds its feet and tail underneath, close the 

 body inferiorly, and give it the form of a spheroid. The pos- 

 terior extremity of the tail is without appendages. {Typhis 

 oroides, Risso.) 



Anceus, Risso, Gnathia, Leach, 



Which have also the thorax divided into as many segments, 

 as pairs of feet, but in which all those organs are simple and 

 monodactylous. They have, besides, four antennae (setace- 

 ous.) The head is strong, squared, with two large projections 

 in the form of mandibles. The extremity of the tail has some 

 foliaceous appendages in the form of fins. {Anceus forjicula- 

 ris, Risso.) 



Praniza, Leach, 



Have four setaceous antennae, as well as Anceus ; but the 

 thorax seen above presents only three segments, the first two 

 of which, very short and transverse, have each one pair of 

 feet, and the third, much larger and longitudinal, bear the 

 others. All the feet are simple. The head is triangular, 

 pointed in front, with the eyes projecting. The posterior ex- 

 tremity of the body also presents a fin on each side. {Oris- 

 cus cceruleatus, Montagu.) 



