1855.] Linnean Society. 429 



and certainty the points of distinction from all those which are 

 already known, and a description so full as to enable the future ob- 

 server to ascertain whether any individual afterwards examined is 

 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 referring 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 ova to- orbicularis, subglobosa, lasvis, polita ; 

 fronte subproducto, fossulas antennarias tegente. Orbita fissuris tribus. 

 Fossa antennaricB obliquae, apertse. Pedipalpi externi caiile exteriore 

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

 acute triangulari. Pedes antici crassioi-es, longitudine mediocres, 

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

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

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

 coalitis. 



1. Leucosia Urania, Herbst. Test^ subglobosa, antice producta, fronte 

 rotundato ; brachiis triedris, supra ad basin tubei-culis paucis ; sinu 

 thoracico usque ad latera regionis hepaticze antice attingente, granis 

 marginato. 



2. Leucosia cra?iiolaris, Linn. Testa ovato-rhomboidea, fronte tridentato ; 

 brachiis serie tuberculorum ad latera et tuberculis duobus tantum ad 

 basin. 



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

 tice circular!, tuberculis circumscripto ; brachiis ad latera et ad basin 

 tuberculatis, manibus longioribus quam latioribus, serie granorum ad 

 raarginem interiorem. 



No. LXV. — Proceedings of the Linnean Society. 



