Xxii INTROCUCTIO^f. 



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 Brj'ozoa, 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 palaeozoic 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, 

 tiiat 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 Ormoccras, 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. Suflicient 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 arc 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 palaeozoic strata, arranged in the 

 chronological order of those strata. The clear and undisturbed succession which prevails 

 tliroiighout (lie 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. 



