JEFFERSON COUNTY. 
391 
No. 5, is a portion of the head of a calymene, merely the central part. It is given in the 
state and form in which it is usually found; hence it serves, even in this condition, to cha¬ 
racterize the rock. 
No. 6. Ceraurus pleurexanthemus. This is more rare than the preceding; and from its 
strong resemblance to a calymene, I have doubted whether the description given by the Palas- 
ontologist of the Survey may not require some modification. Whether this may be so or not, 
being a trenton fossil, it will serve the purpose for which it is introduced. The figure repre¬ 
sents merely the head, the other portions being rarely found. 
No. 7. Trinucleus tessellatus, {Cryptolitlius, Green), is another crustacean extremely abun¬ 
dant in some localities; thus, multitudes may be procured from the limestone near Glen’s- 
Falls. I did not observe it at Essex, Plattsburgh or Chazy. There seems to be the same 
relation in the distribution of fossils, as in that of animals at the present day. They are accu¬ 
mulated in localities which were favorable to their habits when living. Hence, though a fossil 
may be confined in its range to a single formation — a rock, yet it does not follow that it will 
be found in every location of that rock. In many instances, great multitudes of the crusta¬ 
ceans herded together, having the instinct of sociality ; though the fact may have arisen from 
the favorable conditions of the location, the supply of food, concealment, protection from 
violent currents and uncongenial temperatures, together with the various other considerations 
which influence animal life. 
No. 4. I have introduced three figures of a recent crustacean, for the purpose of illustrat¬ 
ing the preceding fossils. I have placed them side by side with those ancient ones which 
abounded so much in the Trenton era, and which, if we may place reliance upon the faithful 
observations of hundreds of geologists, whose efforts have been directed for years to this sub¬ 
ject, are proved to have become extinct with that era, as none of the same beings appear 
above the Champlain gi’oup. It is obtained from pools left by the receding tides along the 
shores of Cape Horn. This small crustacean appears closely allied to No. 3 of Fig. 100. 
From the observations of Dr. Eights, to whom I am indebted for the specimens, both species 
have the same habit of bending their bodies, and bringing their extremities into contact, in 
which position they are always found when left by the tide. Indeed, it appears that most of 
the genera of fossil crustaceans indulged in this habit, as we often find them thus coiled or 
bent. The texture, too, of the superior plates of the body and extremities, is as similar as 
possible to the petrifactions of the trilobites. We never find but a very small part of the 
inferior surface of the fossils of which I am speaking, so that many facts in relation to them 
remain unexplained ; their mode of progression and of rest; whether they adhered to bodies 
by suction or by legs, etc. On these points, it is barely possible that discoveries hereafter 
may throw some light. It is probable that the inferior surface was uneven, and that the mat¬ 
ter in which they are enveloped adheres in such a manner that it is impossible it should be 
detached so much as to show the inferior structure. Now we see that this recent trilobite has 
only feeble legs, short and obscure ; and hence if an animal thus constructed were to be 
imbedded in mud, and subsequently consolidated, there Avould be a very small chance of 
