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ECHINOCONUS 



Agassiz made a cast of this specimen for his collection of moulds of fossil Echinoderins. 

 This form, therefore, has become classical, and I prefer to retain it, with Mantell's name, 

 as M. Cotteau has done, in his beautiful work on the ' Fossil Echinides of the Department 

 of the Sarthe.' 



Echinoconus subrotundus is a common Urchin in the Lower English Chalk. The 

 subcircular outline being rounded anteriorly, and a little contracted posteriorly, some- 

 times is slightly sub-pentagonal, as in the large specimen figured in PI. LIII, 

 fig. 2 a, b ; the upper surface is elevated and convex, or sub-conoidal. A fine series of 

 Avell-preserved specimens gives about equal numbers of both forms. The base is narrow 

 and flat, and much rounded off at the border; the sides are tall and convex, not much 

 inflated. Mr. Bone has given admirable figures of this species in Pis. LII and LIII, 

 with full details of the anatomy of the test. 



The ambulacral areas are narrow and lanceolate, PI. LII, fig. ] b, c, PI. LIII, 

 fig. 2 a, b, and composed of small plates, of which five are opposite one large inter- 

 ambulacral plate. PI. LIII, fig. 2 e, representing a portion of the large test near the 

 ambitus magnified four times, shows the structure of the ambulacra ; two rows of tu- 

 bercles occupy the area, and form thereon irregular oblique rows ; the poriferous zones are 

 extremely distinct ; the pores are very small, one oblique pair corresponding to each plate. 



The inter-ambulacral areas are nearly three times the width of the ambulacral, and 

 formed of wide deep plates (PL LII, fig. d, PI. LIII, fig. 2 e). In the large specimen 

 (PI. LIII, fig. 2 c, d) there are fifteen plates in each column between the border and the 

 disc, and six or seven between the border and the periostome ; each plate supports three 

 or four longitudinal series of tubercles arranged in quincuncial order, fig. 2 e, each sur- 

 rounded by an areola, and having its summit perforated; the tubercles on both areas are 

 very small, indistinct, and nearly homogeneous, and can only be seen with a lens, so that 

 the shell appears quite smooth to the naked eye. The inter-tubercular surface is covered 

 with microscopic granulets. 



The base is flat, PI. LII, fig. 1 a, PI. LIII, fig. 2 b, sometimes it is undulated from 

 the convexity of the basal inter-ambulacra, and the tubercles are a little larger than those 

 on the sides ; PI. LIII, fig. 3, represents the arrangement they assume on three basal 

 plates magnified six times ; the bosses are crenulated, and the tubercles perforated. 



The mouth-opening is small and central, about one seventh the diameter of the base ; 

 and the periostome presents very feeble indications of entailles (fig. 2 b). 



The vent is larger than the oral opening, and occupies the border (Pis. LII and LIII) ; 

 it has an elliptical form, placed vertically, the most acute angle being directed upwards 

 (PI. LIII, fig. 2, b, d) ; the opening is on the same plane with the border of the test, and the 

 single inter-ambulacrum is neither tumid or rostrated, like B. abbreviatus (PI. LIII, fig. 1). 



The apical disc is firmly soldered to the surrounding plates, and is almost always well 

 preserved ; it is small, and consists of five ovarial plates, four of which are perforated, and 

 one single imperforate, the antero-lateral plate is large, and projects into the centre of 



