FROM THE INFERIOR OOLITE. 



293 



Hyboclyfus agakiciformis. Forbes, in Morris's Catalogue of British Fossils, 2d edition, 



p. 82. 

 NucLEOLlTES DECOLLATUS. Quenstedt, Handbuch der Petrefactenkunde, tab. 50, 



fig. 6, p. 585. 

 Galeopygus agakiciformis. Cotteau, Mem. lu a la Soc. Geol. de France, le Juin, 1856. 

 — — Desor, Syiiopsis des Echinides Fossiles, p. 167. 



Test in general disciform, sometimes convex or conical, with a pentagonal circum- 

 ference ; ambulacral areas narrow, the three anterior straight, the posterior pair sinuous ; 

 inter-ambulacral areas wide, unequal, the plates covered with a great number of microscopic 

 tubercles; apical disc small, central, and vertical; anal valley deep, with parallel sides, 

 which gradually expand about the middle of the single inter-ambulacrum ; mouth opening 

 small, sub-central, nearer the anterior border; peristome feebly decagonal; poriferous 

 zones narrow, pores unigeminal in the upper surface, at the base wide apart and 

 trigeminal. 



JDimenaions. — The measurements given in the following table are made from four 

 specimens which represent the four varieties this species assumes. 



No. 



Hyloclypus agariciformis, Forbes. 



Height. 



Transverse 

 diameter. 



Antero-posterior 

 diameter. 



Inches. 



Inches. 



Inches. 



1 



2 

 3 



4 



Disciform var. a . . . 

 Type var. /3 . . . 

 Convex var. y . . . 

 Conical var. 6 ... 



0^ 



1 



1^ 



H 



2t% 

 2^^ 



3A 



2f 



2t>^ 



2A 



Description. — In the above table I have given the measurements of four specimens of 

 this urchin, with the view of showing the diversity which exists in the relative proportions 

 of diflPerent forms of the same species. Were an observer to find only Nos. 1 and 4, he 

 would be almost justified in considering them distinct species ; but when he discovers a 

 number of intermediate forms, by which the depressed, hemispherical, and conical varieties 

 are seen to blend into each other, he is convinced that they all belong to one and the same 

 species, and at the same time is taught the important lesson, that mere difierence in 

 form and bulk alone do not constitute a specific character. 



This fine urchin, the oldest of the genus, is the largest Hyloclypus we are acquainted 

 with ; it has a sub-orbicular or sub-pentagonal shape (PI. XXI, fig. 1), and is the 

 most typical form of the discoidal variety No. a. It is expanded and depressed above. 



