24 MEMOIR II. 



That the Natural History of the Opossum Shrimps may 

 be rendered as complete as the state of our knowledge 

 permits, and for the satisfaction of the Scientific Naturalist, 

 and the Systematist, the following short description of the 

 Genus has been drawn up, which may be passed over by 

 the general reader. 



The Corselet or cephalo-thoracic-clypeus, resembles that 

 of the Prawns, without however being remarkably pro- 

 longed in front. 



The Eyes are very large, spreading, and on rather long 

 pedicles. 



The Antennce or feelers, consist of an inner and an 

 outer pair ; the former, arise from between the eyes, are 

 composed of three robust basil joints, of which the upper- 

 most is short and supports two long multi- articulate setae, 

 the innermost of these setse is shorter, and carried straight 

 in front, wliile the outermost spread out in a lateral 

 direction ; the latter or outer pair of antennae, are placed 

 upon a lower level than the former, originate from the 

 inner side of the anterior lamina or scales, and end in a 

 single long multi-articulate seta, extended douTiwards and 

 outwards by the animal in swimming. 



The anterior Lamina, or scales which accompany the 

 outer antennae, correspond Avith the same members in the 

 Shrimps and Prawns, but are longer in proportion, and 

 vary in their shape, so as to furnish characters for the 

 distinction of the species. 



The Mouth, situated as in the Shrimps below the base 

 of the antennae in front, is provided with a labrum, and 

 with a bilobate under lip, a pair of palpigerous toothed 

 mandibles, and two pair of complicate foliaceous maxillae or 

 jaws. The Palp has its first joint much abbreviated, the se- 

 cond and tliird, broad, and strongly pectinate on the margin. 



Feet: Unlike all the other Macroura, (in which the 

 three anterior pair of feet are disguised and appropriated 



