the Crinoidese and the Echinodermata. 283 



are distributed through the strata greatly aids the geologist in his 

 inquiries. They enable him to identify at distant points rocks 

 which may perhaps present different appearances of mineral 

 composition and whose geological position is but obscurely in- 

 dicated, but which, when the imbedded fossils are carefully 

 examined, prove them to be of contemporaneous age; for we 

 know by experience that certain genera and species of animal 

 remains occur in regular and beautiful sequence in the different 

 groups of rocks, and that in many cases certain species are 

 peculiar to a single formation, by which it can be distinguished 

 even amidst the greatest confusion ; that is, when the original 

 arrangement of the strata has been so disturbed as to reverse the 

 order of their superposition, or their complications by faults so 

 great that every relation to distant masses is rendered obscure 

 and doubtful. 



In the Crinoidese, Mollusca, and other remains of ancient and 

 by-gone periods, we find so many beacon-lights, or directing 

 points, that, by an accurate knowledge of genera and species, we 

 can arrive at results and conclusions, that under other circum- 

 stances we could never hope for, and but for such knowledge 

 could never be attained. With a view of contributing to this 

 desirable end, we have undertaken the task of describing the 

 Crinoidese in a monograph, not only because their remains are 

 found so abundantly in the older formations, but also on account 

 of the many new forms which have been discovered, and which 

 have thrown so much additional light on a subject formerly so 

 obscure and complicated. 



Since those early periods in the earth's history when the Cri- 

 noidese existed in countless myriads, they have gradually dimi- 

 nished in numbers during subsequent ages, until only a few 

 species are found amongst the living creation ; and these, with 

 the exception of one, the Pentacrinus Caput Medusae, are so un- 

 like ancient forms, that few persons, except the scientific, would 

 identify them as allied to this once abounding tribe. 



To the geologist, the zoologist, and those who desire informa- 

 tion, the fossil genera present a wide field for inquiry and contem- 

 plation : — their symmetry of form, and the beautiful arrangement 

 seen in the elaborate contrivance by which hundreds of thousands 

 of separate indurated pieces are so placed that each piece is so 

 nicely fitted to the adjacent pieces, that a skeleton seemingly so 

 complicated in its mechanism becomes obviously, as we study the 

 uses of the various parts, as simple in action as it is delicately 

 articulated and geometrically constructed. 



In the Eoctracrinus Briareus I have counted upwards of 741,710 

 joints or separate pieces, exclusive of the small plates which stud 

 the membrane that covers the inside of each ray, and which 



