UIKTACRINUS: ITS STKUCTURE AND EELATIONS. 33 



In many cases the centrale of Form M is much smaller than the combined 

 infrabasals and centrale of Form D, in specimens of corresponding size ; and 

 it is sometimes smaller than the centrale alone. For instance, how could 

 the centrale of such specimens as Nos. 249 c, 12, 15, I w, 250 a, and 60 a 

 (Table B) have been produced by the fusion of infrabasals and centrale in 

 specimens like Nos. 40, 88, 42, 47, 69, and 43 ? As the table shows, the 

 average for a number of adult specimens is, for Form M, centrale = 1.28 

 mm., while for Form D, the infrabasal circlet = 2.35 mm. 



As I have already stated, there is no evidence in the specimens of any 

 fusion among the plates. Whatever tendency can be detected in the fossil 

 condition is rather the other way. In the more mature specimens of Form D, 

 in which the plates have become much thicker and firmer than in the young, 

 the sutures among the infrabasals, and between them and the centrale, not 

 only become more distinct than is frequently the case among young speci- 

 mens, but they often appear somewhat separated, or disturbed among them- 

 selves, through pressure in fossilization. Examples of this are given on 

 Plate II., where Fig. ]2 shows the infrabasals disturbed and partly raised out 

 of position, and in Fig. 13 they are w^ell separated. But the most complete 

 case is that of Fig. 3 on Plate VIL, where I have given an enlarged photo- 

 graph of the base of No. 247, with the infrabasals lying entirely apart from 

 each other and from the centrale. If there is any tendency among the 

 plates of the dicyclic base, exhibited by the fossils, it is to become more 

 distinct and well marked by growth. 



Even this would not end the difficulties. Mr. Bather, by a beautiful 

 series of observations and illustrations,* has demonstrated that the presence 

 or absence of infrabasals is correlated with the radial or interradial position 

 of the lobes of the chambered organ, which lodges the nerve centre co- 

 ordinating the muscular movements of the whole skeleton. That is, if 

 the base be monocyclic, the lobes of the chambered organ correspond 

 with the basals, and are interradial ; and if the base be dicyclic, the lobes 

 of the chambered organ correspond with the infrabasals, and are radial. 

 Hence, as he says, " the derivation of one type from the other involves 

 more change than the mere atrophy or appearance of certain plates." 

 Now, according to this, in Form D of Umtacrinus, whether old or young, 

 the lobes of the chambered organ would be radial, and the branches of 



* Geol. Mag., Decade IV., Vol. V., pp. 422-425, Sept., 1898; also Ray Lankester's Zoology, Chap. 

 XI., p. 104. 



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