58 SVEN LOVEN, ECHINOLOGICA. 



two obloiig, high and spacions excavations, /"o., fig. 52, 53, 56, 

 58, Fl. VIII, fig. 66, arched över the auricles and the motory 

 mnscles, and reaching sometimes to near the -upper surface 

 and the dental slide. 



The sides of the pyramid, fi.g. 59, are the two extended 

 and nearly flat lateral surfaces, that give attachment to the 

 inter-pyramidal mnscles, fig. 52, 57, lam. int., 58, 59, 60. 

 They agree in size and form. Each of them is marked by 

 concentric curves of aceretion, and divided, by a faintly sig- 

 moidal row of large holes rnnning from the labial process 

 to the middle of the external margin, into an upper and 

 a lower area. From that row there radiate, npwards and 

 downwards, like the two vanes of a feather, two series of 

 slightly elevated ribs pierced by regnlar rows of smaller holes. 

 There are no transverse ridges like those in the Regnlaria 

 for the attachment of the muscles, fig. 68. The lateral sur- 

 face is a thin sheet, lam. int., connected with the compact 

 main body of the pj^ramid in the following manner. 



As in all tlie dentate Echinoids, the density of the calcified 

 tissue of the pyramid varies in different parts, Fl. VIII, fig. 

 66 — 72. It attains its highest degree of hardness and is all 

 but homogeneous in the two strong ridges stretching from the 

 labial process, one along either side of the middle body, Pl. 

 VII, fig. 58 X, 59 X, Pl. VIII, fig. 06 x . Almost equal in so- 

 lidit}' is the reticular textnre of each half- pyramid at the 

 mesial symphysis. It there forms a distinct vertical zone 

 consisting of regnlar layers of very minute meshes, Pl. VIII, 

 fig. 66 X X. 67 x , disposed longitudinally and, less distinctly, 

 transversely. It is ver}' narrow at its npper margin, where 

 it gives rise to the mesial ridge, increases downward in 

 thickness, by mnltiplying its layers, and is rather strong 

 in the septnm of the fosste, fig. 58, 66. Inwards it continnes 

 towards the dental support, of which it forms an essential 

 part, Pl. X fig. 110, 116, 119, 120, 130. All along the sym- 

 physis this compacted tissue is closely accompanied in each 

 half-pyramid by a rather irregular network, fig. 67, which 

 has the appearance of being derived from it, but assumes, 

 somewhat abruptly, a transverse direction and a looser consi- 

 stence. The meshes become wider and stretched in the trans- 

 verse direction, fig. 68, 69, their sides are drawn ont into fi- 

 laments connected by minute bars, and finally collect together 



