363 Linnman Society : — 



new, or identical with the one so described. Specific characters, the 

 author says, should always be either absolute, or derived from 

 points of comparison within the individual itself. 



After a general view of the structure, it is shown that from a low 

 degree of development of the organs subserving the functions of rela- 

 tion, these animals are necessarily slow and circumscribed in their 

 locomotion, and dependent for their safety from injury upon external 

 means of protection. 



In refen-ing to the distribution of the family, it is shown that 

 every genus, without exception, is restricted to its own geogra- 

 phical limit ; and this is true to such an extent, that there is no in- 

 stance of one species of any genus inhabiting the old world, and 

 another of the same genus being found in the new. 



The systematic portion of the paper consists of the characters of, 

 and observations on, all the genera whether new or previously known, 

 and characters and descriptions of every known species. Five new 

 genera and no less than thirty-six new species are described. The 

 following are the generic and specific characters as given in the 

 paper : — 



Genus Leucosia. Testa ovato- orbicularis, subglobosa, Icsvis, polita ; 

 fionte subproducto, fossulas antennarias tegente. Orbita fissuris tribus. 

 Fossa antennar'ue oblique, apertce. Fcdipalpi cxterni caiile exteriore 

 lateribus parallelis, recto vel subcurvo, apice obtuso ; caule interiorc 

 acute triangular!. Pedes antici crassiores, longitudine mediocres, 

 brachiis ad basin et ad latera tuberculatis. Abdomen maris, in non- 

 nuUis speciebus, segmentis omnibus primo et ultimo exceptis — in aliis 

 tertio cum quarto et quinto cum sexto ; fixmmce a tertio ad sextum 

 coalitis. 



1. Leucosia Urania, Herbst. Testa subglobosa, antice producta, fronte 

 rotundato; brachiis triedris, supra ad basin tuberculis paucis; sinu 

 thoracico usque ad latera regionis hepaticse antice attingente, granis 

 raarginato. 



2. Leucosia craniolaris, Linn. Testa ovato-vhomboidea, fronte tridentato ; 

 brachiis serie tubeiculoruni ad latera et tuberculis duobus tauluni ad 

 basin. 



3. Leucosia obtusifrons, De Haan. Fronte rotundato, sinu thoracico an- 

 tice circulari, tuberculis circumscripto; brachiis ad latera et ad basin 

 tuberculatis, nianibus longioribus quani latioribus, serie granorum ad 

 maigineni interioveni. 



4. Leucosia nnidentata, De Haan. Fionte unidentato; sinu thoracico 

 antice circulari, tuberculis perlatis circumscripto: brachiis facia supe- 

 riore seriebus binis tuberculorum. 



5. Leucosia rhomboidalis, De Haau. 'J'csta rhomboidali, antice producta, 

 multo longiore quani latiore; brachiis basi utrinque dense tomcntosis, 

 lateribus tuberculatis, supra plerumque Iravibus. 



6. Leucosia longifrotis, De Haau. Testa subglobosa, fronte producto, in- 

 tegerrimo ; sinu thoracico antice elliptico, granis non cincto ; brachiis 

 ad latera tuberculatis, et granulis paucis ad basin. 



7. Leucosia orbicularis, Bell. Testa orbiculari, fronte lato, brevissimo, 

 bidentato ; sinu thoracico nullo ; sterno antice grauulato. 



