32 GEOLOGICAL SURVEY 
them to have such. The classification of recent crustacea stands in the same relation to fossil 
species that Cuvier’s classification of recent fishes stands to those that are found in a fossil state. 
The hard covering of these animals does not admit of extension with the growth and enlarge- 
ment of the body. 'T’o obviate this the crust or shell is cast off, and it is probable that the usual 
imperfect state in which these fossils are found is due to the fact that they consist, for the most 
part, of these empty crusts. 
Desmarest felt the difficulty of establishing the analogy between fossil and recent species, 
where those parts were wanting upon which their distribution in the system depended. He showed 
that the protuberances and inequalities of the exterior of the shell were connected with the struc- 
ture and organization within, and by a careful study of these, that a classification might be estab- 
lished that would afford the Paleontologist results as satisfactory as those derived from the classifi- 
cation of fossil fishes by their scales. Whoever will follow up the method indicated by this 
naturalist will confer a vast benefit on this department of Paleontology. 
The class Crustacea has been separated into two groups—those with the eyes situated on 
moveable peduncles, and those in which they are immovable. 'To the former belong crabs, 
shrimps, &c. and to the latter the trilobites, which are all extinct. fora long time these fossils 
were regarded as insects, and were even placed in that class by Linneeus himself. 
The anterior portion of the body, to which the head belongs, is composed of a crescent shaped 
plate; in the middle is situated the abdomen, which is composed of plates laid over each other, like 
the tail of a shrimp or lobster, and articulated in such a manner, in some species, as to allow the 
animal to roll up like the common wood-louse. This portion of the body is divided by two grooves, 
more or less distinctly, into three lobes, and hence the name. Some species have a tail composed 
of a semi-circular plate. 
The most extraordinary circumstance connected with these animals is the preservation of their 
eyes, which are found to be highly organized; and in Asaphus caudatus each eye is said to contain 
400 lenses. This appears truly wonderful when we reflect that these animals were among the 
earliest inhabitants of the primeval ocean, and gives but little countenance to the doctrine of 
development. 
No feet have yet been observed in these animals, so that it is probable that those organs only 
existed in a soft and highly perishable state. Some of them are surrounded by a border very 
similar to that of the Chiton, which may have served the purpose of attaching them to the rocks 
in the manner of that animal. But where the head is distinct and the organs of sight well 
developed, we must suppose that the animal possessed iocomotive powers. 
They existed in great numbers in the early silurian seas, and ceased to exist during the carbonif- 
erous period, 
Dr. Emmons has figured a crustacean, found at Cape Horn by Dr. Kights, which has very much 
the habit of a Trilobite. When left in the pools along the shore after the recession of the tide it is 
found rolled up in the manner in which the fossil Trilobite of the genus Calymene so often oceurs. 
The following are the principal genera of the Trilobites. 
Calymene, Bumastis, 
Asaphus, Odontocephalus, 
Homalonotus, Crypheus, 
