32 SMITHSONIAN INSTITUTION 



of Ichthyocrinus they are found completely resorbed (Pis. XXXV, figs. 4b, 5; 

 XXXVI, figs. 3a, b). In Plate XXXV, figure 5, it will be observed that the 

 lumen of the axial canal, instead of being radial as usual in the genus, has with 

 the disappearance of the infrabasals become interradial in position. The same 

 thing occurs in Cleistocrinus, where in one specimen the resorption involves 

 part of the basals also (PI. XXXVIII, fig. 2b). In the two Carboniferous 

 species of Euryocrinus the infrabasals are partly or wholly resorbed (PL XL, 

 figs. 4a, yb), while in the Devonian representative of the genus they are intact 

 (PL XL, fig. id). In Amphicrinus, the latest survivor of the Flexibilia, the 

 infrabasals are often partly, and in some cases entirely, resorbed; and here we 

 have the converse of the occurrence in Ichthyocrinus, for while the axial canal 

 in the genus normally has its angles interradial, in three specimens where the 

 infrabasals have completely disappeared the lumen has become radial (PL XL, 

 fig. gb). Thus in these cases there seems to be a change from a dicyclic to 

 monocyclic base within the same species, accompanied by a corresponding 

 change in the orientation of the axial canal. It is noticeable that in these cases 

 of resorbed infrabasals there is usually an open space in the position they would 

 have occupied larger than is required for the stem lumen. A similar fact is 

 seen in the Recent Endoxocrinus parrae, which in contrast to other closely re- 

 lated forms of the Pentacrinidae has no infrabasals. 



An extraordinary modification of infrabasals in the opposite direction 

 from that of the Ichthyocrinidae is found in the Silurian genera Homalocrinns 

 and Calpiocriniis, in which these plates are enormously enlarged, so that they 

 envelop, and in some cases completely conceal, the basals and even the radials 

 (Pis. VI, figs, ic, 2b; VIII, figs. 3, yb, c). This is a condition which does not 

 exist in any other crinoid. I first called attention to it in 1906, 1 before which 

 date the structure of these two genera had been wholly misunderstood ; and the 

 full details will be found later on in the appropriate generic discussion. 



The nervous system of the crinoids is centered in the chambered organ, 

 which in some of the Flexibilia was lodged in a peculiar funnel-shaped cup on 

 the inner floor of the infrabasals, presently to be described. That the seat of 

 vitality was located at this place is proved by a case of recuperation in a speci- 

 men of Taxocrinus, where the calyx had been broken away in life leaving only 

 the infrabasals and one basal, from which a new crown was produced with six 

 rays (PL LVI, figs. 11a, b, c). In genera like Ichthyocrinus of which the in- 

 frabasals, having lost their original function, were too small to accommodate 

 such a funnel, it moved up to the basals (text-fig. 10). A similar structure is 

 found, variously modified, in specimens belonging to the orders Camerata and 

 Inadunata, both monocyclic and dicyclic. 



'Journal of Geology, vol. 14, p. 480. 



