G 



BRITISH GRAPTOLITES. 



communicate witli each other at their bases, and open to the exterior at their distal 

 ends {apertures). These subcylindrical thecffi are usually somewhat compressed 



Fig. 1. 



Fig. 2. 



Nenia. 



Theca 1^ - 



Growth-lines of tli. 21 



Common canal 

 iperture of tU. R' 



Denticle of tli. 4i 



Crossing cana!- 



.. /' ' V 



Aperture of sicnia- 



]* 



.'• » 



Fig. 1. — Didymograx>tus hifidus (Hall). 

 Fig. 2. — Didymograptus hifidus (Hall). 



V 



Dorsal edge Ventral eJge 

 ofSndstipe. of2ndsupe. 



n J = arc of angle of divergence. 



Diagrammatic figure of proximal portion, obverse view. 

 Diagrammatic figure of proximal portion, reverse view. 



X 5. 

 X 5. 



into the form of more or less quadrangular tubes, each of which may be conven- 

 tionally regarded as bounded by four walls, viz. one dorsal (inner), one ventral 

 (outer), and two lateral walls. The distal free margins of these walls constitute 

 collectively the apertural margin, the plane of which is almost invariably at right 

 angles [normal) to the axis of the theca itself. The corner (or coign) made by the 

 common edge of the lateral and ventral walls and the apertviral margin occasion- 

 ally forms a well-marked denticle, and is sometimes prolonged into an apertural 

 spine. The ventral wall of each theca — which is conventionally regarded as 

 defining the length of the theca itself — is in contact with [overlapped by) the dorsal 

 wall of the theca immediately preceding for a variable portion of its leng-th, but is 

 free towards its distal extremity. In well-preserved specimens the Avails of the 

 thecae are marked by closely set lines of growth, which run parallel with the 

 apertural margins. 



The vast majority of examples of Grraptolites occur in a compressed state, 

 forming a thin flattened film upon the surface of the rock. In this condition 

 the most characteristic appearance of the polypary in Didipnograjjtas is that 

 presented by the profile view, in which the dorsal edge is seen to be smooth and 

 continuous, and the ventral edge denticulated by the outer margins of the thecee 

 and the apertural margins. 



In one aspect [obverse) of the profile view of the polypary the sicula is visible 

 for the whole of its length ; in the opposite aspect [reverse) the sicula is more or 

 less concealed by the crossing canal. The stipes originate from the sicula by 

 means of a bud developed from its left side (when the polypary is regarded in the 

 typical or obverse aspect), and from this bud two sets of structures are developed, 

 namely — 



(1) The first theca (th. V) and the first so-called common canal, which give 

 origin to the first stipe by a process of budding ; and 



