jan. — mar. 1857.] voyage from England to Madras. 159 



June 3rd, Lat. 3125 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 part 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 

 Hyalceas. 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 Hyalaa, that it is impossible to 

 draw the line between them, 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. 8 7 "9 E. a Utile 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 

 Byalcea 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, 1 1 ; 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: 



