156 ANNALS NEW YORK ACADEMY OF SCIENCES 



Eesult: Through forms like irregular -e, liomplirayi, generic evolution 

 into Desmograptus up to Devonian; one-half per cent of our forms, 

 because of somewhat later starting of evolution. 



Good stolon in later species, sessile; finally, thinning of rhabdomes 

 (tenuiramosus, a sessile ^seudo-Dendrograptus) . 



IV. Var. ruedemanni. Heavy branching and a successive diminution 

 of the elastic cross-threads produce a thick brush and dense crowding of 

 nourishing thecse, while still allowing movement of the rhabdomes. 



Eesult: 35 per cent of whole fauna; 50 per cent perfectly preserved. 



Half of the observed attaching organs belong to this variety. Evolu- 

 tion: (a) persistence of type in a few Ordovician and Silurian species; 

 stolons and roots common. Compare delicatulum, perexile, rectilinea- 

 tum, siibretiforme, tenellum, filiramum. (b) Further reduction of 

 cross-threads, enforced reduction in size. Intermediate forms: Callo- 

 graptus, Rliizograptus, Odontocaulis, Callyptograptus; sessile, in early* 

 stages vagrant; then Dendrograptus; stolons and roots. Further evolu- 

 tion of Dendrograptus leads to thin flexuose types (i.- e., under slight 

 pull) or to succulent types (i. e., under heavy pull) ; latter forms some- 

 times with central axis. End of this evolution: Ptilograptus, Acantho- 

 graptus, etc. 



V. Var. conferta, cylindrical growth and close network, giving me- 

 chanically the best type. 



Eesult: On one side 6 per cent of our forms and 70-80 per cent per- 

 fectly preserved; on the other no specific evolution at all, probably on 

 account of very unfavorable conditions of food supply. 



VI. Last type of change by thickening and dividing of the cross- 

 threads (groups of peltatum, cervicorne, cavernosum, tuberosum) not 

 represented in our fauna. Appears first in American Ordovician. Sto- 

 lons and roots well developed. 



Considering this mechanical starting point, it becomes of interest that 

 the Grorgonias of equivalent habitat (disregarding the funnel shape of 

 the Dictyonema colony) present the same lines of changes as those 

 sketched in the foregoing for Dictyonema flabelliforme. Thus we find 

 among the Leptogorgias, L. eximia and media with a network like that of 

 var. acadica, Leptogorgia agassizi with close fine meshes comparable to 

 those of var. conferta, while Leptogorgia rigida looks like a callograptoid 

 type and Eugorgia multifida like a Dendrograptus with beginning central 

 axis. The latter, still further developed, gives Pterogorgia acerosa, 

 while Oorgonia flabellum has a typical desmograptoid appearance and 

 Gorgonia quercifolia resembles closely the norwegica-murrayi line. I 

 cannot help thinking such a conformity indicative of parallelism in 



