116 COELENTERATA—CNIDARIA CLASS II 
extremely in different species. In some forms it is circular or quadrate, and 
upwardly directed ; in others it is contracted and opens inferiorly, when the 
hydrothecae are bent downwards. Not infrequently the lower lip is orna- 
mented with one or two slender spines, which often subdivide and inosculate 
with one another. 
The polypary in most Graptolites is furnished at its proximal end with a 
minute, triangular, or dagger-shaped body called the sicwla (Fig. 204), which 
represents the original embryonic skeleton. The first stage of development 
consists in the formation of an axis or virgula along the entire length of the 
sicula, which axis often projects freely at one or both extremities of the latter. 
Hydrothecae are then budded either uniserially along one side, originating from 

Fic. 204. 
a, Monograptus gregarius, Lapw. ‘ Sil- 
urian; Dobbs Linn, Scotland. Proximal 
end showing sicula, enlarged; b, Didymo- 
graptus pennatulus, Hall. Ordovician 
(Quebee Group); Point Lévis, Canada. 
Proximal end showing sicula, enlarged 
(after Lapworth). 







m| > 
AW 
S 
A 
& a AS 
NY 
Vans P 
2 \ SS 
My SK 
yy NS 
SZ . 
NZ A 
NF) us 
Fic. 203. 
a, c, Climacograptus typicalis, 
Hall. Ordovician (Trenton 
limestone); Cincinnati, Ohio. = 
a, Vertical section, enlarged, 

Fic. 202. 

A, C, Monograptus priodon, showing central virgula; b, In- 
Bronn. sp. Silurian (Etage dividual of the natural size; 
EK); Prague. A, Hydrosoma, c, Cross-section, enlarged; 
natural size. £6, Longitu- d, e, Diplograptus palmeus, Barr. 
dinal section, enlarged. (, Silurian; Prague. Hydrosonia 
Dorsal aspect, enlarged. of the natural size and several 
D, Monograptus Bohenicus, times enlarged (after Barrande) ; 
Barr. Same locality. «a, Vir- Ff, Diplograptus foliaceus, Murch. Diplograptus Whitfieldi, Hall. Sil- 
gula; c, Common canal; th, Ordovician (Llandeilo Group) ; urian. Polypary furnished on both 
Hydrothecae; «#, External Scotland. Natural size (after sides with gonangia, probably dehis- 
aperture (after Barrande). Lapworth). cent. Natural size (after Hall). 
near the major end of the sicula, or in alternate sequence along both of the 
lateral margins of the sicula; but in monoprionidian types the entire polypary 
is developed from the minor extremity of the sicula alone. The sicula itself 
ceases to grow, as a rule, after the first hydrothecae are budded, and sometimes 
it becomes obsolete or absorbed. In many cases it develops into a terminal 
