BIHANG TILL K. SV. VET.-AKAD. HANDL. BAND 18. AFD. IV. N:0 1. 63 



compressed, moderately arcuated, convex towards the slide, Pl. 

 VII, fi(f. 52, 60. The back, TI. X, fig. 133, is marked with 

 a longitudinal, mesial, rounded elevation. The dull, roughened, 

 superlicial layer, x , bordered by the narrow and glossy rims, 

 in the npper part occnpies the whole breadth of the back, 

 then contracts and in the lower part is confined to this eleva- 

 tion. The body of the tooth fills the dental groove, fig. 134, 

 and in the interspace between the two roughened areas, is 

 seen the fibrous tissne, x . The sides of the compressed tooth, 

 the expanded keel, are concave, and, as far as contained in 

 its matrix-sac, roughened and dull, while the crown, that pro- 

 jects into the pharynx, is glossy on all sides. It is not a 

 cuspidate crown as in the Regularia, with the point of its 

 body projecting beyond the keel, bnt the broad prismal mäss 

 of the keel, Fl. Jil, fig. 149, 150, containing bundles of diffe- 

 rent hardness, and thus forming rounded tubercles, is enclosed 

 by the harder lameilar body, and both together present an 

 oblique section, oblong in shape, wedgelike at its inner end, 

 and bevelled oiF on the sides by friction against the other 

 teeth. The tooth 5, Pl. VII, fig. 52, 61, Pl. XI, fig. 149, is much 

 the larger and the crown seen sideways makes a greater angle 

 against its middle line, than in the 2 and 3, and the 1 and 4, 

 which are the smallest, fig. 60, fig. 149. 



Johannes Muller described the dental system of the Arach- 

 noides Placenta L., and gave an exact and correctly oriented 

 hgure of it.' It is lower than that of the Clypeaster reticu- 

 latus L., stellate, Pl. XI, fig. 146, the rays long, the front 

 one, '2, h, o a, slightly produced, the reentering angles much 

 deeper. The sides are bilat era lly unequal; if the 5, which is 

 higher than the others, is taken = 1, the 2 and 3 are = 0,8S, 

 the 1 and 4 = 0,7 4. The elevated figure formed by the crests 

 of the alveoles is not ovate, but pentagonal, unconformable, its 

 salient angles coinciding with the reentering angles of the 

 står. The dental slide is almost rectilinear, inclined at about 

 16°, Pl. VIII, fig. 73, 74. The area of the symphysis has no 

 npper pointed part; its outline is at once diverted exteriorly 

 at an obtuse angle, and in joining the lower almost horizon- 

 tal outline, which is expanded sub-labially, circumscribes a 

 compact hunip half as long as the entire lateral area of the 



Ueber den Bau der Echinodermen, Berl. Abli. 1854, p. 75, PL VII, fig. \'r>. 



