BIHANG TILL K. SV. VET.-AKAD. HANDL. BAND 18. AFD. IV. N:0 1. 57 



in the Echinometra, and it is seen tliat the pyramid of tlie 

 Clypeaster is drawn out exteriorly into a hump, and tliat the 

 greater mäss of it, which in the former lies on the oesopha- 

 geal side, is thrown ontward in the Clypeaster. The pyramid, 

 constituted by the two halves, which are united at the sym- 

 physis by fibroiis tissue, thus becomes expanded horizontally 

 and reclining, not upright, and the whole of the tive pyra- 

 mids forms a comparatively flattened, stellate pentagon the 

 rays of which are ambulacral. Seen from above, /?//. 57, each 

 pyramid has the ontlines of an arrow-head. The two sides 

 are feebly arcuated; the external börder forms a reentering 

 angle; the point, the interiör angle, is truncated and exca- 

 vated över the dental groove. The upper surface, that of the 

 large hump, is slightly coneave, its sides somewhat rising, 

 the lateral wings rather sloping externally, the internal angle 

 strongly raised, and the mesial suture marked with a narrow 

 ridge. It exhibits a distinct division into a middle part, its 

 body, and two lateral parts. The middle part, corp. fig. 52, 

 57, 58, equally divided by the mesial suture, is compact, the 

 lateral parts lamellate. At its inner, truncated end the middle 

 part is raised into a transverse crest er., fig. 57, only a little 

 above the styloid terminations of the dental slide, and ending 

 on either side in a thickened process, pr. sup., fig. 52, 57, 60, 

 the supra-alveolar process, and this is cut down vertically, 

 leaving a ledge on whicli rests the epiphysis fig. 52, 57, 59 

 ep., a simple, flattened, oval plate lower and produced beyond 

 the process interiorly, slightly convex and glossy in its middle, 

 laterally flush with the surface of the pyramid. Very similar 

 in form to either of the two epiphyses, between which it is 

 contained, is the rotula, fig. 57, ro., fig. 62, ambulacral in po- 

 sition, likewise a minute simple plate, pointed interiorly, slightly 

 coneave on the sides, and broad at the upper margin. 



The lower surface of the pyramid, fig. 58, presents the 

 same division into a compact middle body, corp., larger in pro- 

 portion than above, and two lateral more narrow lamellated 

 borders. The middle body is coneave between the projecting 

 borders, and the mesial suture marked by a prominent ridge 

 that strengthens adorally and forms the lower, labial process, 

 pr. lab., which joins the oral termination of the dental slide 

 and in its emarginated termination receives the end of the 

 tooth. Near this point the ridge becomes the septum between 



