On a new Oenus of Amphtpod Crustaceans. 389 



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

 single shoot. 



Shoots of acctcia 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 underground 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 tj-pe. 



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 collateral (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 under 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-surAdval 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 growth. 



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

 leaf-orders was a two-ranked arrangement, some\\hat 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 Biddcll Airy, K.C.B., President, in the 



Chair. 



"On a new Genus of Amphipod Crustaceans." By Eudolph 

 VON WiLLEMoES-SuHM, Pli.D., Naturalist to the 'Challenger' 

 Exploring-Expedition. 



In lat. 35° 47', long. 8° 23', off Cape St. A^incent, 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 fiiceted eyes. The 

 animal, evidently hitherto unknowTi, will be the type of a new 

 genus, having the following characters : — 



