i893. NOTES AND COMMENTS. 251 



the microscope, but the threads were of a quite different structure, 

 consisting of long cells, almost all of the same form, and, according to 

 the author, evidently belonging to a monocotyledon presumably 

 Posidonia, numerous rootstocks of which occurred on the beach mixed 

 with the pine cones. These long filaments had played the part of a 

 thread uniting the smaller debris, which otherwise would probably 

 have remained separate. 



Other cases are mentioned in which the Posidonia fibres act in 

 this way. Bits of sponge may form a nucleus round which they 

 group themselves, while seaweeds may get involved. An example 

 exists in M. Bornet's herbarium, a green alga in the shape of a ball, 

 in which the Posidonia filaments are caught like pins. The latter 

 example is comparable with the instances cited by Masters, and 

 found in certain small English lakes, of Confervae interlacing and 

 forming a ball with the leaves of the larch. 



These recall the so-called aegagropilous algae, which are merely 

 species of the green alga Cladophora, detached at an early stage from 

 their support, and rolled by currents. As Russell suggests, all sub- 

 merged bodies, under certain circumstances, might form similar 

 pellets. The pine needle balls from the lakes of the Engadine are 

 well-known, and M. Jaccart, of the Lausanne Museum, states that 

 in a little creek in the Lake of Geneva, where the water is unceasingly 

 disturbed by currents, fine felted pellets may be seen formed by 

 shavingfs from a saw- mill. 



Evolution of Premolar Teeth. 



That indefatigable worker, Professor W. B. Scott, of Princeton, 

 has contributed to the last issue of the Proc. Acad. Nat. Sci. 

 Philadelphia (1892, pp. 405-444), a very interesting paper on the 

 evolution of the premolar teeth of mammals, especially those of the 

 Ungulates and their allies. It is clearly shown that premolars start 

 as simple cones, and receive their first accression by the addition of a 

 smaller cone on the inner side. It is thus evident that the protocone 

 in these teeth is external ; whereas, according to the general inter- 

 pretation, in the true molars it is an internal element. Professor 

 Scott is thus led to conclude that in Ungulates like the horse, when 

 the upper premolars exactly resemble the molars, the various cusps 

 of these two series of teeth are not homologous with one another. 

 He, therefore, proposes a new series of terms for the premolar cusps. 



The idea that such precisely similar teeth have a totally different 

 origin for their cusps is sufficiently startling to make us anxious to 

 ascertain whether the inductions on which the theory rests are well 

 founded. Now, at the conclusion of his paper, Professor Scott 

 mentions that Herr Rose has recently come to the conclusion that in 

 quadritubercular upper molars the cusps have been wrongly identified, 

 and that the protocone is, after all, external. If this be so, adds Pro- 



