South latitude 20 degrees 52 minutes; west longitude 1 degree 49 minutes.
This day we caught a Velella of the following dimensions: Length of interior cartilage 1.1 inches.
Breadth of interior cartilage 0.5 inches.
Total length of blue base 1.7 inches.
Breadth of blue base 1.0 inches.
Height of centre of crest 0.5 inches.
Rim round crest, in breadth 0.55 inches.
This animal differed from those caught on the 11th November 1837, in the following particulars: It was much larger.

The base of the animal consisted of two parts.

The centre portion was an elliptically-formed cartilage, elevated in the centre, and marked with eighteen concentric striae, which became thinner and thinner as they approached the centre.
No striae were visible on the elevated crest with which the animal swims, but this crest was furnished or fringed with a thin moveable flap, 0.55 inches in breadth, which ran quite round it.

The animal has the power of flapping this to and fro constantly, as a fish does its tail.
The outer portion of the base was of a pale prussian blue colour, increasing in depth of shade both to the outer and inner edges.

Many minute black spots were dotted all over this.