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 

 strono- resemblance to a calymene, I have doubted whether the description given by the Palae- 

 ontolof^ist of the Survey may not require some modification. Whether this may be so or not, 

 beinff a trenton fossil, it will ser\'e the purpose for which it is introduced. The figure repre- 

 sents merely the head, the other portions being rarely found. 



No. 7. Trinudeus tcsscllalus, {Crijptolithus, 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 Uving. 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, conceahnent, protection from 

 violent currents and uncongenial temperatures, together with the various other considerations 

 which influence animal life. 



No. 4. I have introduced tlu-ee 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 group. 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 hght. 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 would be a very small chance of 



