AET. 23 THE ESMERALDA FORMATION ^BERRT 11 



slightly under 1 millimeter. The fourth specimen is an impression 

 and the punctate surface of the matrix shows that the surfaces of 

 the fruit were spinose. The clays are in places packed with the 

 macerated slender linear plant tissues which, in part at least, are 

 believed to represent the foliage of Gerato'phyllu'm. In some cases 

 these are seen to be dichotomously forked, and I have illustrated two- 

 of the clearer of these objects. (Cat. No. 37309, U.S.N.M.) They 

 are obviously incomplete, as is definitely shown by the larger of the 

 two, in which the artist has attempted to indicate the disintegration 

 of the more delicate distal portions of the leaf as it is preserved 

 in the clay. 



The existing hornwort is a gregarious, completely submerged 

 aquatic of ponds and slow streams, which is of rather unusual botan- 

 ical interest, since it is one of the few aquatic vascular plants which 

 has altogether lost the habit of aerial pollination. In Geratophylhj/m 

 the pollen is carried to the surface by the buoyant stamens which 

 then dehisce, and the released pollen sinks slowly through the water 

 until it comes in contact with the stigmas of the ovulate flower. 

 This represents possibly the highest degree of aquatic specialization 

 in a descendant of a terrestrial ancestor and is correlated with root- 

 lessness and the entire loss of vascular tissue in the stem. 



CeratopTm/lluoTh is practically cosmopolitan in the existing floras,, 

 occurring on all of the continents (except Antarctica) and on oceanic 

 islands such as the Bermudas and Fijis, which raises interesting ques- 

 tions with respect to its means of dispersal and possible antiquity .. 

 The fruits sink at once and the plant is soon killed by sea water or 

 exposure to the atmosphere, so that currents can hardly be the agents 

 of dispersal over great distances, which would seem to have been due 

 to the transportation of the seeds by wading birds. 



The systematic position of the genus has also been a mooted point 

 which seems now to be fairly well settled as a much reduced relative- 

 of the Cambombaceae. 



Systematists recognize from one to three existing species based 

 on the presence or absence of the marginal wing and basal spurs, but 

 it must be admitted that a great variability in these features pre- 

 vails. In North America GeratophyUum deniersum Linnaeus is 

 found in appropriate environments everywhere except in the far 

 northern part. Although the in general great geographical range 

 of most aquatics is fully recognized it would seem that cosmopoli- 

 tanism would demand some specific differentiation. The geological 

 history of C eratophylluTTh is exceedingly obscure. 



The known geological occurrences of € eTatophyllmn at pre-Pleis- 

 tocene horizons have been few and unconvincing. Ettingshausen re- 

 corded some stem nodes and very indefinite fruits from the supposed 



