JAN.— MAR. 1857.] voyage from England to Madras. 159 
June 3rd, Lat. 31-25 S. Long. 84-15 E. We took in the net 
several Creseis of a different form from those we had hitherto met 
with, the posterior x^art of the shell terminating in a curved point. 
This is shown in Fig. 25. Fig. 25 a. shows the position of the 
shell when the animal is in motion, which it effects much in the 
manner of those already described except that the convexity of the 
curve is always directed backward, in this respect resembling the 
HyalcBas. And I may here observe, that some authors consider Cleo- 
dora, (of which Creseis and Cuvieria are by them reckoned only 
sub-genera) so closely allied 'to Hijal(£a^ that it is impossible to 
draw the line between the^i, and in support of this anology they 
advert to the fact,that the animals, in their internal structure, differ 
but little from each other. Now although the analogy is undoubt- 
ed, yet the differences seem sufficiently marked to warrant the 
present arrangement of genera and even species, at least the ana- 
logy is not stronger than that observed among other Molluscs, 
the anatomical distinctions between which, are almost impercepti- 
ble, and yet they have been established into species and genera by 
the common consent of Naturalists. It is worthy of observation 
that the three species of Creseis were all captured in daylight, only 
one species, Fig. 23, having been ever taken by us at night. In this 
respect they contrast remarkably with the Hyalceas which, as far 
as we could ascertain, are strictly nocturnal or crepuscular animals. 
We continued working at the net all day, but were not successful 
in getting any more of this species until- 7th June, Lat 26-0 S. 
Long. 87-9 E. a little after 12 o'clock noon, when we took two or 
three specimens alive. In the evening a little after dark we got 
two specimens of Hyalcea Fig. 9. This species much resembles 
Hyalcea Fig. 12 but differs from it in the direction of the lateral 
spines. It is also considerably larger. The animal has the power of 
withdrawing entirely into its shell, but often when at rest it kept its 
fins extended as in Fig. II ; when in motion the triangular mem- 
brane observable between the fins in the above figure was doubled 
over the aperture of the shell as represented in Fig. 10, the dark 
blotches are the viscera of the animal as seen through the shell. 
In the net with these we found a Hyalcea Fig. 17, differing from 
any previously met with in being more angular in form and in 
having the anterior part of the shell deeply grooved horizontally; 
