On a new Oenus of AmpMpod Crustaceans. 389 



complete epitome of phyllotaxy in a single plant, or even iu a 

 single shoot, 



Shoots of acficia often present a zigzag disposition of theii 

 leaves, on either side of the branch, which seems unintelligible 

 except as a distortion of an original two-ranked order. 



The prevalent two-ranked arrangement of rootlets or roots seems 

 to be a survival undergroimd of an order which originally pre- 

 vailed through the whole plant, root, stem, and branch. 



In the whole Monocotyledonous class the first leaves in the seed 

 have the order |. 



In the Dicotyledonous class the first leaves in the seed have 

 the simplest order of the whorled t^^e. 



As the spiral orders have probably been derived from a two- 

 ranked alternate arrangement, so the whorled orders have pro- 

 bably been derived from a two-ranked mllateral (two abreast) 

 arrangement. This is illustrated by an experiment similar to 

 the former ; and it is seen that successive parallel horizontal pairs 

 of spheres are compelled imder contraction to take position at right 

 angles to one another, exactly in the well-known crucial or 

 decussate order. These whorls of two contain potentially whorls 

 of three and four, as is seen in variations of the same plant ; but the 

 experiment does not show the change. 



The reason of the non-survival of the (supposed) two-ranked 

 collateral order lies in its manifest instability ; for under lateral 

 pressure it would assume the alternate, and imder vertical the 

 crucial order. 



The bud presents in its shape a state of equilibrium between 

 a force of contraction, a force of constriction, and a force of gro\vi;h. 



To sum up, we are led to suppose that the original of all existing 

 leaf-orders Mas a t\\o-ranked arrangement, somewhat irregular, 

 admitting of two regular modifications, the alternate and the 

 collateral — and that the alternate has given rise to all the spiral 

 orders, and the collateral to all the whorled orders, by means of 

 advantageous condensation in the course of ages. 



March 6, 1873.— Sir George Biddell Airy, K.C.B., President, in the 

 Chair. 



"On a new Genus of Amphipod CVustaceans." By EuDOLrn 

 VON AViLLEMoES-SuHM, Ph.D., NaturaUst to the 'Challenger' 

 Exploring-Expedition. 



In lat. 35° 47', long. 8° 23', off Cape St. Vincent, the trawl 

 was sent down to a depth of 1090 fathoms on the 28th of 

 January, and brought up, among other very interesting things, a 

 large transparent Amphipod with enormous faceted eyes. The 

 animal, evidi^ntly hitherto unkno\Mi, will be the type of a new- 

 genus, having the following characters : — 



