26 UINTACRIXUS: ITS STRUCTUEE AND RELATIONS. 



h. When the infrabasals are present (Form D), they are radial, and the 

 centrale is interradial : — 



Whereas in pseudomonocyclic and dicyclic forms the outer angles, or 

 pentameres, of the stem — the next element below the base — are inter- 

 radial, whether the infrabasals are visible or invisible. 



In Form D the centrale does not seem in any wise to overlap the infra- 

 basals, as if they were growing out from under it; but it lies distinctly within 

 their circlet, and abuts by its five sides against the inner faces of the infra- 

 basals. In these cases the centrale is often very small, — it and the ring of 

 infral)asals together sometimes occupying but little more than the space of 

 thu larger pentagon when occurring alone. This is not always the case, as 

 there are also frequent instances in which the centrale in the monocyclic 

 form is no larger, actually or relatively, than the same plate in the dicyclic 

 form, and occupies much less space than the infrabasals in specimens of 

 equal size. In fact, the general rule is that the infrabasal ring is larger than 

 the centrale is in monocyclic forms. The average of a number of specimens 

 of 50 mm. width and upwards is, for Form M, centrale =: 1.28 mm. ; and for 

 Form D, centrale = .97 mm., and IBB circlet =: 2.35 mm. The infrabasals 

 form a decagon, or five-pointed star, with more or less obtuse re-entering 

 angles (Plate II., Figs. 10, 11, 12, 13), whereas the centrale in both forms is 

 simply a pentagon with straight sides (PI. II., Figs. 1, 2). The shape of the 

 two is very distinct, and it is difficult to see how either one could have 

 replaced the other under any circumstances, or at any stage of growth. 



In Form M the centrale is subject to much variation in size, ranging from 

 a maximum of 2.75 mm. (PI. II., Fig. 1), to the minimum of .75 mm. (PI. II., 

 Fig. 2), and even less in one specimen. This variation has no definite rela- 

 tion to the size of the surrounding basals or radials, or of the calyx. In 

 specimen No. 60 a, which is one of tlie largest in the collection, the centrale 

 is no larger than in the smallest monocyclic specimens, and is indeed no 

 larger than the minimum size of the centrale in adult specimens of Form D ; 

 and in No. 249 c, an average adult specimen, the exceptionally small cen- 

 trale is not larger than that of small sizes of Form D. The following table 

 shows the variations in the centrale and infrabasal ring in a number of 

 specimens ; the width of the calyx as it lies flattened, and also that of the 

 basal and radial circlets, are also given for comparison. The measurements 

 in this and all subsequent tables .are in millimeters. 



I have not seen an example of Form D in which there was no centrale. 



