Prol. J. Miiller on the Structure oj the Echinodenus. 21 



chambers of the two halves of an ambulacrum may, in the space 

 from the mouth to the petaloid ambulacrum, contain about 

 16,000 pores for the ambulacral vessels of the locomotive feet; 

 if we add to these the 4000 locomotive pores of the petaloid 

 ambulacrum, we shall find that Clypeaster rosaceus has in an en- 

 tire ambulacrum about 20,000, and in the five ambulacra together 

 about 100,000 locomotive pores. About 15 are visible in an 

 area of 1'" in diameter on the external surface. 



The interambulacral plates between the petaloid ambulacra 

 are destitute of pores. The other interambulacral plates, on the 

 other hand, both on the dorsal and on the ventral surface, are 

 porous ; the pores being so arranged that, on the back, they are 

 more distributed upon the outer portion of the interambulacral 

 plates, a poreless area remaining between the perforated arese 

 of any two ambulacra. This poreless area takes up so much space 

 on the dorsum of Clypeaster rosaceus, that 6-10 tubercles arise 

 from it transversely ; on the ventral surface the perforated arese 

 approach so closely, that only 2-3 tubercles stand between 

 them*. 



To the division of Clypeasteridce characterized by ambulacra 

 with double walls and parallel ambulacral chambers from the 

 mouth to the apex, belong, besides Clypeaster rosaceus, many fos- 

 sil Clypeasters whose sections I have examined, as CI. altus and 

 pyramidalisf, and more particularly those high Clypeasters, 

 whose peripheral part is not flattened. On the other hand, flat- 

 tened fossil forms, like CI. scutellatus, M. de S., and its allies, 

 present nothing of this kind, but only marginal chambers, so 

 that it would be justifiable to separate those forms with double- 

 walled ambulacra, under the name of Echinanthus (Leske), from 

 the other Clypeasteridce. 



* Klein's figure (tab. 29) of a section of Clypeaster rosaceus may for 

 the present be sufficient to give an idea of the regular succession of cham- 

 bers, and of the double walls of the ambulacra. 



t There is a Clypeaster in the collection of the Gesellschaft Naturforsch. 

 Freunde with a very high crown similar to the high varieties of CI. altus, but 

 differing totally from it by the breadth of the interambulacral areae between 

 the ambulacra petaloidea, as compared with the ambulacra themselves. 

 The interambulacral area is for half its length, and as far as the upper half 

 of the length of the petaloid ambulacra, about as wide as the internal area 

 of the latter at the same height, and about four times as broad as the di- 

 stance between two corresponding pores ; beyond the upper half of the 

 petaloid ambulacra the interambulacral area is broader than their internal 

 area. In all varieties of CI. altus, to which also the species distinguished by 

 Philippi appear to belong, the interambulacral area between the ambulacral 

 petals is very narrow. The peculiarity of CI. pyramidalis may be as di- 

 stinctly recognised in transverse sections by the high median and transverse 

 ridges upon the internal coat of the petaloid ambulacra, the median ridges 

 being united like buttresses with the ventral wall. The Scut ell a pyramidalis, 

 badly figured by Risso, would appear to belong here. 



