MORPHOLOGICAL PART. 123 



of the tegmen at no time attained the rigidity and large size of that in 

 the Camerata, nor did those of the dorsal cup, with the exception of 

 the anal plates, undergo any material alterations. It was the ventral sac 

 which gave to this group its characteristic feature. The sac in the earliest 

 forms was quite small, but it soon attained such enormous dimensions that 

 already in. the Silurian it constituted the greater part of the calyx. At the 

 close of the Carboniferous, however, it dwindled down almost as rapidly to 

 its former insignificance, so that it is represented in Cromyocrinus, Eiqoacliy- 

 crinus, Erisocrinus, and Encrinus by a very short cone. The respiration of 

 the Fistulata was apparently directly through the test, but only at the 

 posterior side of the calyx. It took place either by means of pores along the 

 sac, or by a madreporite placed in front of the sac. The respiratory pores 

 of the Fistulata only pierce the edges of the plates ; while the water pores of 

 recent Crinoids pass through the whole plate. The mode of respiration of 

 the Larviformia is unknown. They had apparently no openings in the 

 calyx except the anus, but they possessed pores along the arms at the sides of 

 the covering pieces, which may have served respiratory purposes. 



Most of the Ichthyocrinidae have interbrachial plates, which in some 

 species are large and massive, in others small ; and some are regularly 

 arranged, others irregularly. The mouth is opened out, the ambulacra are 

 tegminal, and rest in an integument of very small, ill-formed pieces,^ which 

 extend to the interradial plates in the cup, and those at the sides of the 

 brachial appendages. The small plates form pouches or sacs, w^hich some- 

 times reach to the second or third bifurcation. The median lines of these 

 pouches are occupied by the ambulacra, which converge to the mouth, sepa- 

 rating the orals. 



Here we have among Palaeozoic Crinoids a tegmen, which has all the 

 characteristics of the disk of living species, down to an uncovered mouth and 

 open food grooves, thus demonstrating conclusively that the disk as a ventral 

 structure is not confined to the Neocrinoidea. The discovery of this fact led 

 to entirely new ideas touching the derivation of these groups. Before, it 

 had been supposed that the " vault '' ceased to exist in Neozoic times, or was 

 reduced to the orals. It may now, we feel assured, be considered as estab- 

 lished that the structure of the tegmen in the oldest Palseozoic Crinoids, 



* We carefully removed the arms in several well preserved specimens of Onychocrinus Ulrichi and 

 0. diversus, and in several instances found portions of the ventral disk imbedded in the dorsal cup, lying upon 

 the inner floor of the plates. The disk of these species must have been extremely pliable, and probably con- 

 sisted in part of soft tissues. 



