REV. T. RB. R. STEBBING—CRUSTACEA ISOPODA AND TANAIDACEA. 227 
Genus Tynos, Audorin. 
1825? Tylos, Audouin, Explic. planches Crust. Egypte, p. 287. 
1825. Tylos, Latreille, Fam. Nat. Régne Animal, p. 567 (nomen nudum). 
1829. Tylos, Latreille, Régne Animal, vol. iy. p. 141. 
1831. Tylos, Latreille, Cours d’Entomologie, p. 415. 
1836-40. Tylos, Guérin-Méneville, Iconographie Régne Animal, pl. 31, p. 35 
1840. Tylos, Milne-Edwards, Hist. Nat. Crustacés, vol. ii. p. 186. 
1843. Tylos, Krauss, Sudafrik. Crust. p. 63. 
1858. Tylus, Dana, U.S. Expl. Exp. vol. xiii. p. 715. 
1885. Tylos, Budde-Lund, Isopoda terrestria, p. 272. 
1895. Tylos, Stebbing, Hist. Crustacea, p, 425. 
1896. Tylos, Dollfus, Mém. Soc. Zool. France, vol. ix. p. 550. 
1909. Tylos, Holmes and Gay, Pr. U.S. Mus. vol. xxxvi. p. 376. 
Several other references are procurable by consultation of the above-given 
list. Guérin dates his plate 31 December, 1836, but in the description, 
having in the meantime become Guérin-Ménevyille, he gives a reference to 
observations made by Milne-Edwards on the respiratory organs of Tylos in 
the “journal l'Institut, n. 280, p. 152, 9 mai, 1839.” Notwithstanding the 
numerous notices and several excellent figures which we have in relation to 
this remarkable genus, in one or two respects the details of its structure 
seem still to be rather obscure. All the species seem to be very closely 
akin one to the other, there being nothing apparently to justify the opinion 
of Krauss that the two species which he distinguished at the Cape of 
Good Hope might perhaps require to be placed in a separate genus. There 
is, indeed, a vast disparity in size between his 7. granulatus, which is said 
to reach a length of two inches with a breadth of one inch (50 x 25 mm.), 
and the 7. albidus, Budde-Lund, 7°8 mm. by 2°7-3 mm., from the Nicobar 
Islands, but dimensions of themselves are not sufficient for generic distinction, 
even apart from the gradations of size furnished by other species in this genus. 
As there is only one genus at present recognised in the family, Budde- 
Lund does not separate their characters. Among these he gives, appendages 
of the pleon five pairs; first pleopods wanting; appendages of the four 
following segments haying only one ramus apiece, all branchial ; ramus of 
the first segment, however, even in the male, within produced into a long 
compressed penis ; appendages of the sixth segment forming an open column 
below, with a very small outer ramus at the apex. It will be noticed 
that here Budde-Lund, after saying “ pedes primi paris desunt,” remarks, 
“ramus annuli primi tamen etiam in mare intus in penem longum, compressum 
productus,” implying that at least in the male the appendages of the first 
pleon segment are not wanting. In this latter view he agrees with 
Milne-Edwards, who solved the difficulty of finding only four pairs of 
pleopods, not by saying that the first pair were wanting, but that the fifth 
pair were rudimentary. In his Atlas to the ‘Régne Animal,’ pl. 73 bis, 
