2G8 EPI ASTER 



round, supra-marginal ; base slightly convex transversely ; mouth-opening very near 

 the border, bilabiate, with prominent under lip ; apical disc nearly central, and placed 

 before the vertex. 



Dimensions. — Height, 1-^ inch ; length, 2^ inch. 



Description. — There has been so much difference of opinion about this Urchin that I 

 have given full details of the anatomy of its test in PI. LXIII. 



Lamarck, who first described it, has the following diagnosis : — " Sp. cordato- 

 abbreviatus, convexus, subgibbosus, antice retusus ; vertice elato ; ambulacris quinis, 

 duplicato-biporosis ; ano ovato ;" and refers to the figure in the ' Encyclopedic 

 Methodique/ pi. 156, figs. 4 — 6, as the type of the form. Goldfuss gave in his 

 ' Petrefacta ' excellent figures and an accurate description. Woodward sketched a fair 

 outline of it in his ' Geology of Norfolk,' but mistook it for the common Chalk species 

 Micraster cor-anguinum. Desmoulins, De Blainville, Deshayes, Grateloup, and Desor, 

 all retained the species as established by Lamarck. My late esteemed colleague, 

 Professor E. Eorbes, upheld Woodward's mistake, and mentioned it as a variety of 

 Micraster cor-anguinum in the ' Memoirs of the Geological Survey,' Decade III, PI. 10, 

 whilst oddly enough he had previously described it correctly in his account of the 

 Echinodermata in Dixon's ' Geology of Sussex.' D'Orbigny in his ' Paleontologie 

 Erancaise ' groups M. gibbus with M. cor-anguinum, and perpetuates the old Confusion 

 I have pointed out. 



If we admit that the structure and disposition of afasciole or fascioles form characters 

 of sufficient importance for the establishment of generic groups among the Spatangidce, 

 then, indeed, the Urchin under consideration is not only specifically but generically 

 distinct from Micraster cor-anguinum, for, whereas all true Micrasters have a subanal 

 fasciole, forming a ring placed around the base of the posterior border, one half of 

 which embraces the border, the other half the base, the Urchin before us has no 

 fasciole whatever ; and this anatomical fact has been altogether overlooked both by Eorbes 

 and d'Orbigny, otherwise they would not have confused gibbus with cor-anguinum. 



The build of this Urchin differs from M. cor-anguinum in having a flattened base, an 

 angular ambitus, and a pyramidal upper surface, with steeply-inclined sides, the anterior 

 portion sloping at an angle of from 40° to 50°, the posterior from 25° to 30°, and the 

 lateral sides 35°. In half a dozen good typical specimens these measurements represent 

 a fair average. PI. LXIII, fig. 1, c, d, e. 



The ambulacral areas are narrow, and their petaloidal portions not much depressed, 

 fig. 1 a, c, d, e ; the antero-lateral being one third longer than the postero-lateral pair. 

 The poriferous zones of the petals consist of a pair of holes, the inner being rounder 

 than those in the outer line, and they are connected by a slight depression in the plates, 

 fig. 1 g. Beyond the petals the zones diverge ; and in the anterior pair I have traced 

 the holes to the ambitus, in the posterior pair they have disappeared. The anteal sulcus 

 is feebly marked, and the single petal shorter than the anterior pair ; in the petaloid 



