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THE VOYAGE OF H.M.S. CHALLENGER. 
2. The inconsiderable breadth of the posterior portion ; 
3. The similar disposition of dorsal ribs ; 
4. The shortness of the rostrum or posterior point. 
6. Ilyalsea Isevigata, d’Orbigny. 
]836. Hyalxa levigata , d’Orbigny, Voyage dans l’Amerique meridionale, t. v. p. 110, pi. vii. 
figs. 15, 19. 
1850. Diacria Ixvigata, Gray, Catalogue of the Mollusca in the Collection of the British 
Museum, pt. ii., Pteropoda, p. 11. 
1858. Pleuropus Ixvigatus, A. and H. Adams, The Genera of Recent Mollusca, vol. ii. p. 611. 
I refer this form to Cavolinia longirostris, with which it appears to have most 
affinity : — 
1. In the horizontal direction of the posterior margins ; 
2. In the way the ventral lip extends posteriorly beyond the dorsal ; 
3. In the two inconspicuous dorsal grooves ; 
4. In the curvature of the posterior portion (in Cavolinia longirostris the initial 
portion of the shell is always broken off, but even the direction of the truncature shows 
that the portion which has disappeared would have been much recurved dorsally). It 
is of course not to be imagined that “ Hyaldea Isevigata ” is the caducous portion of 
Cavolinia longirostris, to which only its posterior recurved portion corresponds. 
The geographical distribution of the two forms is the same. Cavolinia longirostris 
is abundant in localities where Hyalsea Isevigata has been found. 
# 7. Hyalsea depressa, d’Orbigny. 
1825. Pleuropus pellucidus , Eschscholtz, Bericht fiber die zoologiscbe Ausbeute wahrend der 
Reise von Cronstadt bis St. Peter und Paul, Oken, Isis, 1825, 
p. 735, pi. v. fig. 2 (male). 
1836. Hyalxa depressa, d’Orbigny, Voyage dans l’Amerique meridionale, t. v. p. 110, pi. vii. 
figs. 11-14. 
1850. Clio pellucida, Gray, Catalogue of the Mollusca in the Collection of the British Museum, 
pt. ii., Pteropoda, p. 14. 
1850. Diacria depressa, Gray, Ibid., p. 11. 
1853. Cleodora curvata, Huxley, On the Morphology of the Cephalous Mollusca, &c., Phil. 
Trans., 1853, p. 42, pi. iv. figs. 4, 5. 
1858. Pleuropus depressus, A. and H. Adams, The Genera of Recent Mollusca, vol. ii. p. 611. 
Souleyet 1 has referred Pleuropus pellucidus to Clio cuspidata, and* Gray to “ Cleo- 
dora curvata ,” Souleyet. These identifications appear to me erroneous, as in my opinion 
Pleuropus pellucidus corresponds to Hyalsea depressa. But as the description and 
figure given by Eschscholtz are equally bad, it is better to ignore entirely the title 
which he has bestowed. 
1 Histoire naturelle des Mollusques Pteropodes, p. 48. 
