118 COELENTERATA—CNIDARIA CLASS ILI 
tastrites, Barr. (Fig. 207). Hydrosoma simple, spirally coiled. Common 
canal very narrow, virgula attenuated ; hydrothecae more or less linear, and 
separated from one another by considerable intervals. Silurian. 
Leptograptus, Lapw. Hydrosoma consisting of two long, filiform, undivided, 
inequal primary branches, with hydrothecae developed from major extremity 
of the common sicula. Ordovician. 
Coenograptus, Hall (Fig. 208). Two primary branches originating from the 
centre of a triangular sicula, curving sigmoidally, and giving off simple branches 
from the convex side at tolerably regular intervals. Ordovician. 
Didymograptus, M‘Coy (Figs. 209, 212, a). Two simple, symmetrically 


2 
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SS 

Fia. 213. 
a,c, Climacograptus tupicalis, 
Hall. Ordovician (Trenton 
limestone); Cincinnati, Ohio; 
a, Vertical section, showing 
common axis in the centre, 
enlarged ; b, Polypary of the 
natural size ; e, Cross-section, 
enlarged. d, e, Diplograptus 

Fia, 211. 
palmeus, ,Barr. Silurian ; 
Dicranograptus Fic. 212. Prague; d, Polypary of the 
ramosus, Hall. natural size; e, Polypary en- 
Ordovician a, Didymograptus Murchisoni, Beck sp. Ordovician larged. f, Diplograptus foli- 
(Hudson River); (Llandeilo Group); Wales. b, Dichogruptus octobrachiatus, uceus, Murch, Silurian (Llan- 
New York (after Hall. Ordovician (Quebec Group); Point Lévis, Canada deilo Group); Scotland. 
Hall). , (after Hall). Natural size. 
developed branches springing from a small axillary sicula. Hydrothecae having 
the form of flattened rectangular prisms, in contact throughout. Ordovician. 
Tetragraptus, Salter (Fig. 210). Major extremity of sicula giving rise to a 
non - polypiferous connecting process (funicle) which sends off two simple 
branches at each end. Hydrosoma bilaterally symmetrical. Ordovician. 
Dichograptus, Salter (Fig. 212, 4). Hydrosoma consisting of eight simple 
