SUB-CLASS I HY DROZOA—GRAPTOLITOIDEA 119 
monoprionidian branches, which originate from as many subdivisions of the 
funicle ; the latter often enveloped in a central corneous disk. Ordovician. 
Dicranograptus, Hall (Fig. 211). Hydrosoma composed of two symmetrically 
developed branches which are coalescent in the proximal, and free in the distal 
portion of their length. Distal ends of hydrothecae isolated and incurved. 
Ordovician. 
Sub-Order B. DIPRIONIDAE. Hopkinson. 
Hydrosoma consisting of two or of four vertical rows of hydrothecae placed back to 
back, with the virqulae coalescing to form a central aais. 
Diplograptus, M‘Coy (Fig. 213, d, f). Hydrosoma 
linear, diprionidian. | Hydrothecae rectangular in 
section, inclined, directly overlapping. Virgula em- 
bedded in a median septum, commonly projecting 
distally as a long fibre, and appearing proximally as 
a radicle. Primordial buds originating along whole 
length of sicular margins. Ordovician and Silurian. 
Climacograptus, Hall (Fig. 213, a, ¢). Like the 
preceding, but with hydrothecae separated and stand- 
ing nearly vertically ; they are furthermore sub-oval in 
section, and distally somewhat contracted. Ordovician 
and Silurian. 
Phyllograptus, Hall (Fig. 214). Hydrosoma._ leaf- 
like, and composed of four uniserial rows of prismatic _ Phyllograptus typus, Hall. 
hydrothecae coalescing along the whole length of their ee ue Pyne 
dorsal margins. Hydrothecae often with two lateral Polyparies of the natural size ; 
) ee b, Ideal cross-section, enlarged 
apertural spines. Base of the Ordovician. (after Hall). 

Sub-Order C. RETIOLITIDAE. Lapworth. 
Coenosark of the common canal giving origin to a biserial polypary. Virgula 
either simple and central, or duplicate and attached to opposite sides of the periderm ; 
the latter is much attenuated, and supported upon a network of chitinous fibres.  Sicula 
absent. 
Retiolites, Barr. (Fig. 215). Hydrosoma undivided, elongate, compressed, and 
tapering toward the extremities. Hydrothecae disposed in rows on each side 
of the axis, inclined, and in contact with one another. Virgulae two in number, 
separated, and attached to opposite sides of the peridermal network in the 
median plane of the hydrosoma; one is rectilinear, the other zigzag-shaped. 
Ordovician and Silurian. 
Stomatograptus, Holm; Retiograptus, Hall; Glossograptus, Emmons. — Or- 
doyician. 
Range and Distribution of the Hydrozoa. 
Of the Hydrozoans in which preservation is at all possible, the Hydrocoral- 
linae are known with certainty as far back as the Upper Cretaceous. During 
the Tertiary period they achieved a somewhat greater distribution, but have 
