MOEPHOLOGICAL PAET. 57 



to one half its normal size^ leaving tlie basal disk exactly like that of forms 

 which are normally without the anal plate. 



It is very remarkable that while in all Crinoids with an unequally tri- 

 partite^ monocyclic base, the smaller plate is located to the left of the anterior 

 radial, this plate in the base of the Blastoids lies invariably to the right 

 (Fig. 6). 



Among monocyclic Inadunata, in most of the genera, the changes among 

 the basals are not so readily traced as in the Camerata, owing to the asym- 

 metry of the radials, which more or less affected the form of the basals. 

 Most of them have five basals, but there are some genera with three and 

 four. Anomalocrimis^ so remarkable for its asymmetry, has five basals, but 

 there is frequently a small additional plate interposed beneath the extremely 

 large left posterior radial. As this plate is only occasionally represented, 

 it is probably not of much morphological importance. Storthingocrinus and 

 Symhathocriniis have three unequal basals, arranged like those of the Platy- 

 crinidae. The Calceocrinidae have three or four basals of a decidedly irregu- 

 lar arrangement. Belemnocrimis is the only known monocyclic genus with 

 ^iNQ basals in which the regular anal plate is represented ; but the plate is 

 quite narrow, and affects but slightly the symmetry of the plates of the 

 proximal ring. In the Hybocrinidse, a large radianal is interposed between 

 the radials, and the basals consist of five plates of a rather irregular form. 



That the smaller number of basals is a modification of the original five 

 plates, is indirectly shown by the fact that in many species of Platgcrinus the 

 plates are so closely anchylosed that no vestige of suture lines can be seen. 

 In some of them the lines are marked by grooves, while in others even the 

 sutures are plainly visible. The sutures are always better defined in a 

 young specimen than in the adult, and in some species can be seen the out- 

 lines not only of three, but of five plates. It is further significant that in 

 all PMgcrini of the Warsaw and St. Louis groups, the suture lines are ele- 

 vated, and transformed into ridges by excessive deposit of limestone matter. 



The case of Pisocrinus is very interesting. The Silurian species plainly 

 show five basals, while those from the Devonian of the Eifel, which have 

 been referred by Schultze to Triacrimis, very often have but three. A care- 

 ful examination of a large number of Eifel specimens leaves not the least 

 doubt that in some of them there are still ^yq plates, while in others of the 

 same species the number is reduced by anchylosis to three. Among recent 

 Crinoids also. Carpenter and others have noticed a fusion of the basals in 



8 



