518 
MOLLTJSCA. 
THE SAGITTA. 
THE SAGITTID^. 
These are little fish-like animals furnished with one or two fin-like organs on the body and a 
broad and usually bilobed caudal-fin. 
They are of small size and swim with 
great rapidity. They have hitherto been 
found principally in the North Sea and in 
the Mediterranean. The name of Sagitta, 
given to these animals, refers to their arrow-like appearance. 
THE FIROLIDiE. 
These are either entirely naked or furnished 
— as is the case with the Carinaria cymhium^ of 
which an engraving is annexed — ^with a small, 
conical, keeled shell, which incloses the intes- 
tinal nucleus. The Carinaria has an elongated 
transparent body, dotted with elevated points, 
and furnished toward the upper part of the 
posterior extremity with a sort of fin, which 
performs the ofiice of rudder ; nearly opposite 
to this, on the belly, is a semicircular fin ; with the aid of this it floats in the water. The 
shells of this genus were formerly known under the head of Venus' SU^jper and the Glass Nau- 
tilus. The C. Mediterranea is abundant in the vicinity 
of Nice. A few other species are found in warm climates. 
THE ATLANTID^. 
In these the shell is spiral, and so large as to contain 
the whole animal when it is contracted. These were 
formerly supposed by some naturalists to be the original 
of the fossil ammonites, or at least an analogous family. 
In the Atlanta Peronii, found in the vicinity of the Canary 
Isles, the shell is minute, glassy, and compressed. In the 
annexed engraving, the figure a shows the actual size. 
THE CARINARIA, SWIMMING WITH BACK AND SHELL DOWNWARD. 
THE ATLANTA PERONII. 
Class III. PTER€>POI>A. 
This term is derived from the Greek pteron, wing, and pous, foot, and is descriptive of these 
little animals, which are furnished with a pair of broad, flattened fins at the sides of the head, 
by means of which they are able to swim with tolerable rapidity, through the open sea, which 
is their favorite abode. They seldom approach the shores unless driven thither by the winds. 
They often crowd the sea in such inconceivable numbers as to color the surface for many miles. 
There are two orders of pteropoda characterized by the presence or absence of a shell. 
ORDER 1. THECOSOMATA. 
The animals of this order are always inclosed in a shell, usually very delicate and of a glassy trans- 
parency. There are several genera, 
of forms diff"ering greatly from each 
other. The shell of the Cleodora 
pyramidata is triangular; that of the 
Limacina ros traits resembles a small 
nautilus in form. The Hyalea tri- 
dentata is globular and transparent, 
with long appendages to the mantle. 
This is common in the Atlantic, 
Mediterranean, Indian Ocean, &c. 
CLiiODORA PTRAMIDATA. LIMACINA ROSTRALIS. 
HTALEA TRIDENTATA. 
