HAHN, DIGTYONEMA-FAUNA OF NAVY ISLAND, N. B. 151 



I cannot help feeling that all these facts urge the conclusion that in 

 the history of one large part of the ancient hydroids, there was one route 

 of evolution prescribed, namely : The change of habitat starting from a 

 holoplanctonic and passing through a partly epiplanctonic to a definitely 

 sessile mode of existence. 



On the Occurrence and Stratigraphical Eange oe the Varieties 

 of dlcttonema elabelliforme 



Because, fortunately, large slabs up to a size of 2 square feet were col- 

 lected, I was enabled to attend to the association of the Dictyonema 

 faunula. This is imbedded in an extremely homogeneous, highly car- 

 bonaceous black shale of. finest grain. Small concretions of iron pyrites 

 are frequently scattered over the surface, though infiltration of the fossil 

 material is rarely to be found. The more profusely the animal mass lies 

 buried in the slates, the thinner they split. Cleavage of any considerable 

 amount rarely appears, though often recognizable in minute pseudo- 

 striation running in the same direction over all rhabdomes of a slab. 

 The slates are exactly of the same character as those of the Biplograptus 

 geminus — zone (Llandeilo) from Fogelsang (Skane, Loc. E 5 of No, 

 40 of the Guide of the International Geol. Congress, 1910), those of the 

 Upper Graptolite-shale (Tarannon) from Stommen (Westergoetland) 

 or those of the upper Hartf ell-shale (Caradoc) from the Moffat district 

 (zone No. 8, South Scotland), except that most of the European occur- 

 rences have been a little more affected by stress. 



Two different kinds of embedment can be noticed. Slabs on the sur- 

 face of which single specimens with remarkably fine preservation are 

 found, accompanied by clusters of early stages, alternate with others 

 which are completely covered with a pell-mell of fragments and indi- 

 viduals, often almost indeterminable and lacking any favored direction. 

 Here the variety ruedemanni and Staurograptus are extremely common. 

 On a single slab 1.5 feet square, I numbered more than one hundred 

 neanastic and ephebastic dendromes, omitting the rhabdomes pieced. As 

 a rule, both sides of the colony are closely pressed together without any 

 interbedded layer of mud, a strong evidence of rapid sedimentation, 

 caused by rough and sudden events- On the contrary, the slabs of the 

 first-described mode frequently show a rather thick sheet of mud inserted 

 between the walls of the dendrom. Sometimes this is imbedded at a 

 distinct angle of up to 5 degrees, found between the axis of the colony 

 and the surface of the split shale which is likely to be caused by the- 

 weighing down of the heavier, distal end of the body. A rather slow 

 deposition has to be assumed in these cases. 



