LOrxANOGRAPTUS. 81 



^riie ]i()l_v])ai'y is very siiiiiliii' in its mode ol' dcvclopiiu'iil to l)iclinijr(i/)liis, but 

 possesses capacity for still greater dieliotoni}' ; and hence, vvliile Ih'rlKKjrd/ifns is 

 characterised l)y the possession of stipes of three ordei-s only, LinjtduKjrKj/lus lias 

 stipes of four orders. Thus, in addition to the shoi't JJiih/vingraptns and Trlra- 

 graptns stages, there is a short-lived Dichograptas stage, each stipe of which quickly 

 undergoes dichotomous division, resulting in the formation of the sixteen main 

 stipes of tlie Loganograptas polypary. 



It is not the actual number of stipes, but the degree of capacity for dichotomy 

 which is the essential characteristic of the genus ; thus specimens having seven or 

 six stipes may be true Loganograpti, but there cannot be fcAver stipes than six. A 

 typical Loganograptus has sixteen stipes, all of the fourth order; l)ut there may be, and 

 often is, considerable atrophy. Examples are known showing this in various degrees, 

 such as twelve-stiped, ten-stiped, seven-stiped, etc. ; but, so far as known, the series 

 is not quite complete. In a seven-stiped Logaiiograjdus all the stipes of the second 

 order are developed, but only two undergo further division, so that only four stipes 

 of the third order are produced ; of these only one undergoes further division. The 

 number of stipes is therefore made up as follows : 



2^ + 3^ + 2' = 7 



Similarly, in a six-stiped form, three out of the four stipes of the second order 

 persist as single stipes ; the fourth divides dichotomously, giving rise to two stipes 

 of the third order, one of which persists, and the other is dichotomously divided, 

 the formula being — 



:3- 4- r* + 2' = G 



All the forms of Jjogauograptna at present known seem to bo referable to a 

 single species, Ij. Logani, although there is great variation in the apparent 

 width of the stipes, due to the cleavage of the beds and to the method of 

 preservation. 



Loganograptus Logani, Hall. Plate XI, figs. 1 a—g. 



1858. GraptoJUhus Logani, Hall, Gcol. Survey Canada Eep., 1857, p. 115. 



1859. GrajjtoIUhus Logani, Hall, Pal. N. Y., vol. iii, p. 502, figs. 1—3. 



1865. Grax>toliihus Logani, Hall, " Grapt. of Quebec Group," Geol. Survey Canada, dee. 2, p. 100, 



pi. ix, figs. 1 — 9. 

 18G8. Loganograptus Logani, Hall, "Introduction to Study of Grapt olitidaj," N. Y. State Cab. Nat. 



Hist., 20tli Ann. Rep., p. 226. 

 1868. Dichograpstts Logani, Nicholson, Quart. Journ. Geol. Soc, vol. xxiv, p. 128. 

 1898. Loganograpius Logani, Elles, Quart. Journ. Geol. Soc., vol. liv, p. 476. 



Stipes of first, second, and third orders very short ; main stipes of the fourth 

 order 5 cm. or more in length, somewhat flexuous, narrow at their origin, 

 but widening to a maximum of about PG nun. Theca? eight to nine in 



