Lyman.] 104 [March 7, 



Selaginella among the club mosses, which branches in one plane 

 (Plate 7, fig. 5). 3. If the strong prongs alternate right and left, 

 and if the distances between the forks are unequal, there results 

 unequal alternate forking (Plate 7, figs. 1, 2, 3, 4), and this is the 

 type of Astrophyton, where the strong prongs usually form a nearly 

 straight stem, though sometimes a more or less pronounced zig-zag. 



An inspection of the terminal twigs of an adult Astrophyton, or of 

 the arms of a young specimen, will show that the attempt at first is 

 towards a fork with equal prongs ; and the arm-bone at that point is 

 found broadened into two equal faces corresponding to the prongs. 

 Presently one prong gets larger than the other, and thenceforth 

 maintains its superiority, although both continue to lengthen and to 

 make new forks. At every fork, therefore, there is a strong and a 

 weak prong ; and these succeed each other in alternation ; that is to 

 say, if the second fork has the right prong strong, the third will have 

 the left prong strong; the fourth, the right again, and so on. 



The first fork of an adult, lying near the edge of the disk, seems 

 to split the arm in equal halves ; but the arm of a y oung specimen 

 shows that even in this case one prong is stronger than the other, 

 and grows more rapidly (Plate 7, fig. 1). Plates 4, 5 and 6 show 

 the development of the right-hand prong or stem from the first fork 

 in the arms of Astrophyton costosum, Agassizii and asperum. At the 

 second fork the stem throws a strong branch to the right, especially 

 conspicuous in costosum, where it furnishes two hundred and fifty- 

 three forks in a total of ten hundred and twenty-four (Table u), and 

 in Agassizii, where it has two hundred and twenty-seven out of five 

 hundred forty-six (Table i). The inner branches having had a 

 longer time to grow, naturally show more forks, but not invariably; 

 thus A. asperum has more in the tenth branch than in the ninth 

 (PI. 6). 



The number of joints in the shaft from one fork to another 

 cannot increase, but their bulk increases very much. If a very 

 small A. Agassizii with a disk 2.2 mm. in diameter, and forks 

 only to the second order in each arm, be compared with a large 

 specimen having a disk of 70 mm., and arm-forks to the twelfth 

 order, it will be found that the length of the shaft between the first 

 and second forks increases about twelve times, and its width thirty 

 times, while the number of joints remains the same. In this propor- 

 tion a single adult joint would occupy nearly double the length filled 

 by all the young joints, which gives an idea of the constant and 

 rapid change taking place in the arm during the life of the animal. 



