5 
DENDROCYSTIS IN NORTH AMERICA 1 
By F. A. Bather 
Illustration 
Page 
Figure 2. Dendroct/stis t?) paradoxica Billings 8 
Messrs. A. 0. Thomas and H. S. Ladd 2 have recently published a 
careful description of Iowacystis sagittaria n.g. et. sp. from the Fort Atkinson 
limestone of the Middle Maquoketa series at the old Fort Atkinson quarry, 
Winneshiek county, Iowa. The Maquoketa beds are generally referred to 
the middle of the Richmondian (Upper Ordovician of some authors, 
Lowest Silurian of others) and probably correspond to the upper part of 
Marr's Ashgiilian in Great Britain. 
Iowacystis is referred by the authors to the Anomalocystidae, though 
they admit that certain characters “are foreign to that family/’ and add that 
“Foerste suggests relationship to the Mitrocystidae or Lagynocystidae.” 
It is, however, plain that the species is a Dendrocystis , and, thus considered, 
it may be diagnosed as follows: 
Dendrocystis sagittaria (Thomas and Ladd) 
A Dendrocystis with thecal outline sub-triangular, width at base nearly 
equal to thecal height; thecal lobes descending slightly below stem-attach- 
ment; sides slightly convex, marginals more developed on reverse side, 
3 lateral, 1 apical, and 2 basal, and between the last on each side is a basal 
or adcolumnal plate; these frame about 40 irregular plates on obverse side, 
with greatest diameters from 1*5 to 4*5 mm., and 6 plates on reverse side, 
with greatest diameters from 3 mm. to 6*5 mm., 5 of which are symmetri- 
cally arranged. Most of the plates show obscure axial folds or ridges. 
Length of brachiole unknown; width at base about 1*2 mm. [?]. Length 
of stem uncertain, but at least twice thecal height; section throughout 
elongate-elliptical; distal region of short subalternate dimeres; median 
region of alternate dimeres, with height about twice breadth, and some 
intercalated plates; proximal region short, of narrow flanged dimeres forming 
about eight rings, with no smaller plates. 
Dendrocystis sagittaria occurs at about the same horizon as D. scotica, 
but it is the end (so far as known) of a different line of evolution. The 
European line leading from D. barrandei to D. scotica shows a gradual 
increase in bilateral symmetry, with the brachiole moved to one side and 
counterbalanced by an antibrachial process. 3 Starting again from D. 
barrandei , we must suppose an American line, in which an expanded 
bilateral symmetry continued to affect the proximal region, but was 
replaced in the distal (adoral) region by the more central position of the 
brachiole and the median apical (i.e. adoral) marginal. 
1 Published by permission of the Trustees of the British Museum. 
* Univ. Iowa Stud. Nat. Hist-, XI, No. 8, pp. 6-10 (June, 1926). 
* See Bather, 1913: “Caradoc. Cyst. Girvan.” Trans. Roy. Soc. Edinburgh, XLIX, ii, p. 374. 
