DIDYMOGRAPTUS. 7 



(2) A tubular body (the crossing canal) which grows across the sicula from left 

 to right, forming the first part of the second tlieca (th. P) and the second common 

 canal, from which similarly the second stipe is developed. 



The succeeding tliecse (th. 2\ -V, etc., and 2^, S'', etc.) in both stipes are 

 developed in a continuous linear series, each individual apparently budding from 

 the tlieca immediately preceding it in order, the budding orifices remaining 

 permanently open as the common canal. 



The various species of the genus Duhjmograptus arrange themselves very 

 naturally in morphological groups around certain special forms, each of Avliich may 

 conveniently l)e regarded as normal or typical for a single group. Some of these 

 groups, again, agree in the possession of common charactei'istics of general habit, 

 which allow of their being united in a connected series. How far these groups and 

 series are significant of the phylogenetic relationships of the individual species 

 comprised within them, must be left for future discovery to determine. 



Rei'lined series. 



Iteflcxed series. 



Horizontal series. 



'^''^^^^JSSSJSs Deflexed series. 



ecliued series. 



Deprndfiit series. 



Fig. 3. — Diagrammatic ftgm-e, showing- the relative angles of divergence of the stipes characteristic 



of the several series of Didymograpti. 



Gropp I.— 



-Type B. extensus. 





Group IV. — Type D. v-fradus. 









'D. extensus, 





ID. v-fradus. 



Deflexed 







D. nitidns, 



M 



,, var. volucer. 



series. 







D. uniformis, 



.2 



^D. deflexuH. 







. 



D. patulus. 





Group V. — Type D. Murchisoni. 









D. hirundo, 

 D. sparsus, 



13 



o 





(a) Sub-group of D. hifidus. 

 'D. Murchisoni, 





Group II.- 



-T 



D. pennatulus. 

 ^pe D. superstes. 



1 





„ var. (jehiitius. 

 D. hifidus, 

 VZ). amplus. 

 (b) Sub-group of D. indentus. 

 CD. indentus. 



De- 



Group III 



( D. superstes, 



{D. euodus. ' 

 — Type D. affinis. ^ 





- 



. pendent 

 series. 







^Z>. affinis. 







D. nanus, 



-I 









D. gracilis, 

 D. acutidens. 



OQ 



s 





D. artus, 



^ ^D. .^tabilis. 









D. Nicholsoni, 



^1 



Group VI. — Type D.fasciculaius. ^ 



Eeflexed 







„ var. planus. 

 D. serratulus, 





D. fasciculatus. j 



series. 







D. simulans. 





Group VII. — Type D. ijibberuhis. 



Eecliued 







.D. dr. filiformis. 







D. (jibherulus. 



series. 



