116 



COELENTERATA CXIDARIA 



CLASS ir 



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 arid 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 sicula (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 



A, 0, Monograptus i>r!nilini, 

 Bronn. sp. Silurian (Etage 

 E) ; Prague. A, Hydrosoma, 

 natural size. 11, Longitu- 

 dinal section, enlarged. C, 

 Dorsal aspect, enlarged. 

 1), MniHiiir<ii>tu.s Bohemians, 

 Barr. Same locality, a, Vir- 

 gula ; f, Common canal ; th, 

 Hydrothecae ; x, External 

 aperture (after Barrancle). 



FIG. 203. 



U, C, Clhiificiiiii'iijifiif! tiiiiii'ii!!*, 

 Hall. Ordovician (Trenton 

 limestone) ; Cincinnati, Ohio. 

 a, Vertical section, enlarged, 

 showing central virgula ; 1), In- 

 dividual of the natural size ; 

 c, Cross - section, enlarged; 

 (I, i', I>iplw/raptiis2><tlmeus, Barr. 

 Silurian ; Prague. Hydrosoma 

 of the natural size and several 

 times enlarged (after Barrande) ; 

 /, Diploijritptniifoluice'us, Murch. 

 Ordovician (Llandeilo Group) ; 

 Scotland. Natural size (after 

 Lapworth). 



FKJ. 204. 



a, Monograptus greyarius, Lap\v. Sil- 

 urian ; Dobbs Linn, Scotland. Proximal 

 end showing sicula, enlarged ; /;, l)'nl innn- 

 gmptus pennatuhts, Hall. Ordovician 

 (Quebec Group) ; Point Levis, Canada. 

 Proximal end showing sicula, enlarged 

 (after Lapworth). 



FIG. 205. 



])iplo(jmptus WhitfieliU, Hall. Sil- 

 urian. Polypary furnished on both 

 sides with gonangia, probably dehis- 

 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 monoprionidiari 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 



