PLATYCRINID^. PLATYCRINITES. 17 



It is (juito evident that if the work of disintegration is going on in a rock, the con- 

 tained fossils are mostly in a state of decomposition also, though the progress of decay is 

 generally less rapid in the organic remains than in the rock in which they are imbedded. 

 The almost certainty that decomposition takes place simultaneously in the matrix and 

 its inchided fossils, though in different degrees, would have led a less sanguine observer 

 than Miller to suspect the real cause of some of the variations in the form of the crinoidal 

 fragments which he describes so minutely ; but he had ah-eady embraced another view 

 of the subject, that of the muscularity of the whole calcareous frame work, and conse- 

 quently he appears to have rejected every kind of evidence which militated against this, 

 his cherished theory. 



MiUer also mentions that on yomig undeveloped specimens tubercles are frequently 

 seen. We have never been fortunate enough to meet with any of these undeveloped 

 specimens which ^liUer and other writers delight to aUude to, when describing minute 

 imperfect specimens; for all those wliich have come under oiu- observation, however 

 dimmutive they may be, are as perfect in every pomt of structure as then- larger and 

 full grown congeners. 



Reference to oui- fii-st plate will at once shew that the smaller specimens are not 

 wanting in any essential organ, and that they do not differ in any particular, except 

 size, from the larger individuals of the same species. 



If the variations in form are not, as we suppose, mostly the effects of weathering, but 

 are truly structural modifications, the difficulty still remains as to the species to which 

 they may be properly referred ; for they have ne\ev yet been found under circumstances 

 sufficiently explicit to warrant the conclusion that they appertain exclusively to the 

 P. Itevis. 



Internal and Membraneous Parts. 



Thougli in fossil species no portion of the softer and more perishable parts of the 

 animal remain, by which we can study its internal conformation, or the organization of 

 its membraneous appendages, we can, by comparison with its living analogues and a 

 careful examination of the preserved skeleton, arrive at an obscure Imowledge of its 

 muscular organs. Thus, fi-om the form and arrangement of the tentacida, we consider 

 them to have been furnished mth membranes which became amplified into lateral 

 fimbriate transparent webs, which both aided the animal in capturing its food, and 

 enabled it to move with greater fi-eedom within the limits of its circumscribed sjihere of 

 action. Numerous cii-rhi were probably ranged along the inner siuface of these mem- 

 branes, adding, by their irritability, to the already highly diffused sense of touch which 

 the animal possessed. 



