HAHN, DICTYONEMA-FAVNA OF NAVY ISLAND, X. B. 147 



preserved specimens, the reflecting layer is found beneath the matt ex- 

 terior, which usually adheres to one side of the split slab, the reflecting 

 layer remaining on the counter side. If we shave the reflecting mass 

 away, again a matt black skin can be recognized. This is apparently the 

 same type of observation made by Freeh on the graptolites of Sadewitz. 20 

 He states that a dark horny cover is separable from a brilliant interior 

 layer and that the latter consists of calcite crystals. I could not get, it 

 is true, any reaction on touching the mass with hydrochloric acid. 

 Nevertheless, I believe that the two sheets described may represent the 

 epidermal and mesodermal tissues of these ancient hydroid zoaria. 



As to the composite character of the rhabdomes, on all better pre- 

 served branches, the wrinkled structure long ago recognized by a great 

 many authors can easily be seen in var. acadica and conferta, while rarely 

 observable in the other varieties. After much search, I succeeded in 

 finding a few dendromes on which pyritization had partly taken place, so 

 that several minute tubes about .10 mm. in width are shown in relief 

 running along the thecse in a slight curve (compare Euedemann (1), p. 

 607, text figure 28) toward the aperture. 



The terminal pores of these little tubes, however, were to be found 

 only on one flanking branch of a rhabdome belonging to var. acadica, 

 which closely resembled Wiman's Dictyonema rarum 21 (pi. 12, fig. 10). 

 I was able to trace back this composite structure up to the second theca, 

 but it was by no means recognizable either on the sicula or on any adhe- 

 sive organ. That, furthermore, the wrinkling of the surface has nothing 

 to do with any stress, is proved by the fact that a true cleavage some- 

 times does exist, cutting the widely scattered rhadomes on a slab in one 

 and only one direction. It produces a very fine cross striation which, in 

 the beginning, I was inclined to regard as a true ornamentation; but 

 when E. Euedemann kindly called my attention to the possibility of the 

 other explanation, a renewed study soon brought me to accept it. 



Another old observation, that below the fan-like tissue of a Dictyo- 

 nema, the counter wall of the funnel can be obtained by careful prepara- 

 tion, which I always found true among my material, was elucidated in 

 another way, because I got several specimens which had been quickly 

 buried and compressed in such a manner that the apertural spines of the 

 thecse of one wall make their appearance in regular, pointed rows between 

 the meshes of the covering side of the funnel. Yery often, too, the 

 variety ruedemanni has its rhabdomes so badly pressed together that it 



20 p_ Frech and F. Roemer 3 Lethsea geognostica. part 1, p. 570. 1880-97. 



21 Carl Wiman : "Ueber die Graptoliten," Bull. Geol. Inst. Univ. Ups., Vol. 2, pi. 12, 

 fig. 10. 1895. 



