BRANCHIOPODA. 141 



cessary to imitate Leach informing a separate genus Lepidu- 

 rus, Leach for those which have a lamina between the threads 

 of the tail. Such is the Apus prolongatus; Monoculus apus, L.; 

 Schaeff.,Monoc, VI; Limule serricaude, Herm., Jun.; Desmar., 

 Consid. LII, 2. The carina of the shield terminates posteriorly 

 in a small spine, which is not seen in the Apus canciformis; Bi- 

 noclea queue en filet, Geoff., Insect., XXI, 4; Limulus palustris, 

 Mull.; Schaeff., Monoc. I V; Apus vert, Bosc; Desmar., lb., 

 LI, 1; the latter, besides, has no lamina between the caudal 

 threads; it is the type of the genus Apus, Leach, or the Apus 

 properly so called. The same naturalist has figured another 

 species, Apus Montagui, Edinb. Encyclop. Suppl. I, XX. 



ORDER II. 

 P^CILOPODA. 



The Psecilopoda are distinguished from the Branchiopoda 

 by the diversity in the form of their feet, among the anterior 

 of which an indeterminate number are ambulatory, or fitted 

 for prehension; while the others, lamelliform or pinnate, 

 are branchial and natatory. It is principally, however, by 

 the absence of the usual mandibles and jaws that they are re- 

 moved from all other Crustacea. Sometimes these parts are 

 replaced by the spinous haunches of the first six pairs of feet ; 

 and sometimes the organs of manducation consist either of an 

 external siphon in the form of an inarticulated rostrum, or of 

 some other apparatus fitted for suction, but concealed or 

 slightly apparent. 



Their body is almost always, either wholly, or for the 

 greater portion, invested with a shell in the form of a shield, 

 consisting of a single plate in most of them, and of two in 

 others, which always presents two eyes when those organs are 

 distinct. Two of their antennae Cheliceres, Lat. form a 

 forceps in several, and fulfil its functions. Most of them have 



