ER INTRODUCTION. 
species in which this character is developed, has the depression on one side, as in Caninia. 
Indeed, admitting the subdivisions among the groups as proposed, we shall scarcely find a 
well characterized species of Cyathophyllum in Silurian strata. Of the other families of 
Corals, or the Bryozoa, less is known, but there are nevertheless some important differences 
in character in the ascending development. 
Passing over the intermediate classes, I have noticed, in the OrTHOcERATA, some pe- 
culiarities of structure which appear confined to species belonging to the lower division of 
the system. The first of these is in the undulating septa and angular form of Gonioceras ; 
which is the first instance, within my knowledge, of undulating septa in a straight cham- 
bered shell of paleeozoic date. The singular internal structure exhibited in the Endoceras, 
showing in fact the viviparous character of the animal, is still more remarkable, and 
exhibits a feature in the physiology of these animals before unknown. The great numbers 
of specimens examined leave no doubt of the true nature and object of this peculiar de- 
velopment of the siphuncle, which embraces the embryo sheath. I regret to say, however, 
that I have not been able to make investigations to such an extent as would enable me to 
ascertain the true characters and gradual development of the young shell as it is found in 
the embryo sheath, and subsequently in its free state. Thus far, I have not discovered in 
my collections a single species having this character, from any strata above the Hudson- 
river group. The peculiar character of siphon observed in Ormoceras, I am induced to 
believe, is also found in one or more species in the higher strata; but of this I cannot yet 
be entirely certain. 
This brief introductory notice can scarcely be regarded as more than a slight and im- 
perfect sketch of the subjects introduced. Sufficient progress has not been made, to speak 
with confidence of every point. So many new developments have taken place in the course 
of investigation, that we are ready to expect others in our future progress; and although 
these may not essentially modify general results, they may nevertheless tend to give 
greater force or importance to some minor points, or to show more clearly the untenability 
of others. 
The plan of the work, as has already been stated, is to give a complete representation 
of the succession among the organic forms typical of the paleozoic strata, arranged in the 
chronological order of those strata. The clear and undisturbed succession which prevails 
throughout the entire series, has given a certainty in the direction and result of our labors ; 
and it remains only to be hoped that the same facilities in the following portions will give 
that part of the work an equal value for its accuracy in this respect. 
