BIHANG TILL K. SV. VET.-AKAD. HANDL. BAND 18. AFD. IV. N:0 1. 41 



lialf-pyraraids which are lield together by an intervening fibrous 

 tissue, Pl. X, fig. 129. Seen from witbin, c, each half presents 

 its somewhat triangulär wing, a/ö, slightl}' concave, tbin at 

 the oesophageal, nearly straigbt, margin and transversely 

 truncated superiorly; and externally, a massive arcnated walL 

 npwards continued in a process tliat rises above the wing, the 

 processus snpra-alveolaris, various in form but generally three- 

 sided and more or less pointed. It is to the lateral surface 

 of this process that the main body of the epiphysis is ap- 

 plied, while its basal prolongations rest on the transverse ledge 

 formed by the truncated and groved thickness of the wing. 



Within the alveolar cavity, enclosed between the two wings 

 and the outer wall, the tooth, enveloped in its matrix, is placed 

 on its slide, in the mesial groove, the )sulcns dentalis» Valen- 

 tin, which is equally divided between botli half-pyramids, Pl. 

 X, fit). 110, 111. 113, 114, 115, 116, 120, 122, 123. Each of 

 these has its texture more dense and härd near the symphy- 

 sis. Externally indicated by the narrow line that marks the 

 sutnre, this compacted part of its mäss increases in breadth 

 inwards, so as to form, in the transverse section, Pl. X, fig. 

 110, 116, 119, 120, 130, a triangulär area, the base of which 

 in some species describes a hollow quadrant and, joined to 

 the other half, embraces the nearly semi-circular lumen of a 

 canal occupying the innermost recess of the cavity, somewhat 

 wide above, then contracted. gradually or by means of an 

 oblique ridge, fk). 113, 118, but continued, it will appear, to 

 the lips. On either side of this central canal the alveolar wall 

 presents a longitudinal linear elevation, lincce eminentes pyra- 

 midis^> Val., upwards ending in a more or less prominent sty- 

 loid process, fig. 111, 113, 114, 118, 122, 123, and continued 

 downwards to the mouth. Between its narrow, even, slightly 

 raised margins is included a linear area, fig. cit. x of a some- 

 what duller colour, feebly convex in the Echinus esculentus, 

 fi,g. 122, slightly concave in the Cidaris papillata, fig. 113, ex- 

 tended to the top of the styloid process, and, below, to not fai' 

 from the mouth. These two »lineee eminentes» constitute the 

 slide of the tooth, the bed on which this gradually glides 

 downwards in measure as it wears at the free end. Its con- 

 vex and linear back, smooth and glossy on either side of the 

 mesial groove, closes inwardly the semi-circular canal, fig. 110, 

 116, 120, while each of its sides, feebly concave or convex. 



