252 Prof. J. Miiller on the Structure of the Echinoderms. 



more cover one another than the expanded petals of a penta- 

 petalous corolla; in the closed condition they mutually overlap, 

 just like the folded petals of a closed corolla ; in fact, their 

 sides are quite rolled in. The joints of the coalesced fingers are 

 in general as long as they are broad, or a little longer. 



The dorsal surface of the joints is flat, the lateral processes lie 

 in the middle of the length of the joints, but usually somewhat 

 further forwards, so that the joints viewed from the dorsal sur- 

 face have the form of a cross with very short arms. The union 

 of these joints before and behind, in a longitudinal direction and 

 by their lateral processes transversely, gives rise to a network 

 with regular meshes. At the commencement of the arms the 

 meshes are as yet undeveloped, and the joints not cruciform but 

 four-sided. 



Close above the calyx the joints measure in thickness, that is 

 in the direction from the dorsal to the volar side, much more 

 than in length. The lowest, which rest upon the radialia of the 

 calyx, are the thickest. Thence onwards they decrease suc- 

 cessively in thickness, so that soon it is only one-third of what 

 it was close above the calyx. The great development of the first 

 joints internally, produces, with the radiale of the calyx, a sort of 

 arch over the periphery of the calycine cavity. A nutritive canal 

 is observed upon the articular surfaces at the end of the joints. 

 In a transverse section also we observe that the volar side of the 

 joints is deeply excavated, the cavity being included by two 

 ridges. 



Further outwards the thickness of the joints rapidly dimi- 

 nishes ; an inch from the base they are still twice as thick as 

 they are long, but very soon their thickness is not greater than 

 their breadth. They retain the excavation upon the volar side, 

 which forms a deep canal on the volar side of the longitudinal 

 series of joints, and is covered transversely by minute plates, 

 which usually alternately interdigitate with one another. At the 

 sides of the volar surfaces of the joints, including the ambulacra, 

 stand exceedingly delicate pinnulse or narrow marginal plates, 

 many of which (three to four) occur in the length of a single 

 joint. These pinnulse are unarticulated ; it is only at their base 

 that a small portion appears to be divided oiF. The height of 

 the pinnulse on the broader portion of the hand equals the 

 thickness of the joints. The volar surface of the hand was 

 therefore, corresponding with the dichotomy of the series of 

 joints, provided with hundreds of dichotomous ambulacral 

 grooves, supported by delicate calcareous marginal plates. 



When the series of fingers have been broken out from the 

 stone and the impressions of their volar sides left behind, these 



