54 BEYOZOAN FAUNA OF THE ROCHESTER SHALE. 
Thamniscus dichotomus (Hall). 
Pi. XVIII, figs. 17-19; pi. XXVII, figs. 1-7. 
Hornera ? dichotoma Hall, Nat. Hist. New York, Pal. II. L852, p. 163, pi. 40 C, fig. 3d (not figs. 3a, b). 
Subretepora dichotoma Miller, North American Geol. and Pal., 1889, p. 326. 
Thamniscus dichotomus Ulrich, Geol. Survey Illinois, VIII, 1890, p. <>07. 
Stictopora dichotoma Whitfield and Hovey, Bull. Am. Mus. Nat. Hist., XI, pt. 2, 1899, p. 108. 
At most localities showing me Rochester shale, abundant specimens of an undoubted and 
well-marked species of Thamniscus may be found which has been regarded as identical with 
Hornera dichotoma Hall. In their catalogue of type specimens in the American Museum of 
Natural History", Whitfield and Hovey referred this species to Stictopora with the remark 
that the specimen of Hornera 1 dichotoma which bears Professor Hall's original ticket and 
references to the figures (Nat. Hist. New York, Pal. 11, 1852, pi. 40C, figs. 3a, 3b) is not like 
either description or figures, but is elliptical in section and poriferous on both sides. Through 
the courtesy of Professor Whitfield the writer has been able to examine the type specimen 
referred to and finds it to be a narrow but otherwise normal example of the common Pachy- 
dictya crassa (Hall). Hall's figures, 3a and 3b, are poor and differ in several respects from the 
type specimen. s<> that it is a question whether this example is really the original. They 
might equally well represent a branch of the abundant Thamniscus. Under these circum- 
stances it seem-, best to disregard these two figures and ba.se the species upon the third figure, 
3d, and Hall's description, instead of considering Hornera ? dichotoma as synonymous with 
Pachydictya crassa. As the original description quoted below and the figure on PI. XXVII 
show . Hornera .'dichotoma is an easily distinguished form of Thamniscus, differing from other 
American forms by its extremely regular, dichotomously branching method of growth. 
Original description. Stems minute, semicylindrical, rigidly branching or bifurcating; one surfacl 
st riated longitudinally, the other celluliferous; cells with round or oval apertures, which open from the 
summit of a pustuliform elevation. 
This species is ex1 remely minute, its characters being scarcely distinguishable with the naked eye. in 
its <t riated and poriferous surface it resembles in general characters Retepora; but the mode of branch! 
ing is different, and the cells arc round or oval, with the openings elevated above the surface of the 
branch, which does not occur in Retepora. 
The following descripl ion i> believed to contain the essential characters of t he species: 
Zoarium of small, slender, dichotomously dividing branches generally 0.4 of 0.5 mm. in 
breadth but increasing to 0.6 or 0.7 mm. before a bifurcation. Branching regular, occua 
ringaf intervals of 5 to 7 mm. ; angle of bifurcation small, gen< rally loss than 30°. Branches 
flat on the reverse, slightly convex on the obverse or celluliferous side. Zooecia usually 
arranged in three or four ranges, but jusi before a bifurcation the number may be increased 
to five or six. Zocecial- apertures circular or oval, 0.10 to 0.12 mm. in diameter opening from 
the summits of small pustuliform elevations, <S in 2 mm. measured longitudinally. In young 
specimens the zocecia are arranged in well-marked longitudinal series separated by a low 
ridge, the zocecial spaces are faintly marked with small nodes, and the elevations from 
which the apertures open are pronounced. In old examples the separating ridges and 
zocecial elevat ions are less pronounced while the apertural walls bear numerous nodes which 
often indent the zocecial cavity. The noncelluliferous side is longitudinally striated, the 
lines as shown on well-preserved specimens being composed of small nodes or granules. 
The regular, dichotomously branching method of growth, the flat, striated, noncellu- 
liferous face, and the slightly convex obverse sidewith small rounded zocecia are characters 
which readily distinguish Thamniscus dichotomus from all associated bryozoa. 
Occurrence. — Lockport, Rochester, and other localities in western New York; Grimsby 
and Hamilton, Ontario. 
Catalogue numbers, 35558, 35751, 44147, U. S. National Museum. 
(Bull. Am. Mus. Nat. Hist., XI, pt. 2, 1899, p. 108. 
