286 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



tubes, which often ramify and anastomose." The matter of selecting 

 and fixing the genotype is important for the reason that we refer A. 

 fusiforme to our new genus AUoncuia, while Nicholson and 

 Etheridge's third species, A. xadians, is removed provisionally to 

 Vinclla. 



The restriction of Ascodictyon to species agreeing strictly with the 

 adopted type A. stellatum, renders it a compact and sharply defined 

 genus. As now constituted, the only amendment still possibly defen- 

 sible is the elimination of A. fili forme Vine and A. sparsnni, one of 

 our new species, in which the vesicles, if we leave the connecting 

 threads out of consideration, are always isolated. In all the other 

 species they are normally arranged in radial clusters. If it could 

 be shown that this difference in the arrangement of the vesicles is 

 of genetic significance, we should be strongly inclined to favor a sep- 

 aration. At present, however, we have no direct evidence bearing 

 upon the point and must, therefore, regard the future restriction of 

 the genus as unjustifiable. It may be worthy of remark, however, 

 that even in A. steUatnm at least some of the vesicles of many colonies 

 are isolated or, rather, scattered without definite relation to each 

 other. 



So far as known, the first species of this genus occur in the 

 Silurian, A. silnricnse Vine being fovmd, together with A. fili forme 

 Vine, in the Wenlock shales of England, and the former species 

 also in the Rochester and Waldron shales of this country. The 

 shales of the Hamilton formation also afford two species, the geno- 

 type A. stellatum and a new form for which we propose the name A. 

 Horcalc. The shales of the Chester group is the third and last horizon 

 in which we have detected the genus, and, as in the two preceding 

 cases, here again it is represented by two species, both new. 



ASCODICTYON SILURIENSE Vine 



(Plate LXVIII, ii, 12) 



1881. Ascodictyon stellatum Vine, Quart. Jour. Geol. Soc. London, 

 XXXVII, p. 618. 



1882. Ascodictyon stellatum var. silurieiisc Vine, Ibid., xxxviii, p. 52, 

 figs. I, 2. 



1884. Ascodictyon stellatum var. siliiriense Vine, Ann. & Mag. Nat. 



Hist., ser. 5, xiv, p. 81. 

 1887. Ascodictyon stellatum var. silnricnse Vine, Proc. Yorkshire Geol. 



and Polyt. Soc, ix, p. 184, pi. 12, fig. 6. 

 1892. Ascodictyon siluriense Vine, Ibid., xii, p. 88, pi. 2, fig. i. 



Vesicles pyriform, the small end more or less drawn out, o.i mm. 

 to 0.2 mm. in diameter, and 0.3 mm. to 0.5 mm. in length, arranged 



