J. Hopldnson — Structure of Graptolites. 357 



The branches are only conjoined at the proximal end for the space 

 of two and a half or three hydrothecaa, forming a stem scarcely 

 1-lOth of an inch in length. Commencing in a minute radicle, 

 flanked by two delicate setiform spines, which are but rarely pre- 

 served, it widens out to the axil, and separates into two branches 

 from l-30th to nearly l-20th of an inch wide, and, in the specimens 

 examined, scarcely three-quarters of an inch long. They are nearly 

 straight, and in well preserved specimens are ornamented by minute 

 circular pustules or scars, which appear to mark the junction of the 

 hydrothecse and periderm. The hydrothec^ commence in their free 

 portion with a concave outline, soon becoming convex. They are 

 rounded off at the end, and are sometimes so much incurved that 

 their apertures face directly inwards. 



The branches are frequently twisted until the apertures of the 

 hydrothecse appear on their upper surface, and in this condition very 

 much resemble "scalariform impressions" of Climacograptus Ucornis. 



D. formosus differs from D. sextans, to which it is most nearly 

 allied, in the greater length of its stem, in the smaller angle of 

 divergence of its branches, which are also longer and more robust, 

 and in the elegantly curved form of its hydrothecae. It has a slight 

 resemblance to Didijmograpsus aneeps (Nich.), but may easily be dis- 

 tinguished from it by its possession of an undivided proximal por- 

 tion, by the greater tenuity of its branches, and by the greater num- 

 ber of hydrothec^ in the inch. 



Loo. Llandeilo : — Beld Craig Burn, Moffat, Dumfriesshire. (In 

 black fissile shale.) Llanerchymedd, Anglesea, North Wales (?). 



3. Dicranograptus Nicholsoni, sp. nov. — PI. XVI., Fig. 3. 



Polypary with a short diprionidian stem, much wider near the axil 

 than at the proximal end, and dividing into two monoprionidian 

 branches which diverge from each other at an angle of 70 or 80 

 degrees, and form with each other, at their origin, a continuous curve. 

 Hydrothecse from 20 to 25 to the inch, free for nearly half the width 

 of the polypary ; curvilinear in outline, but not incurved towards the 

 distal end. Apertures extending around the convex distal margin 

 of each hydrotheca. 



The stem is about a quarter of an inch long, and has a minute 

 radicle and two equally minute lateral spines, which are seldom seen. 

 The branches are from l-30th to l*20th of an inch wide, and attain 

 a length of at least an inch and a half. They are joined together 

 at about a tenth of an inch beyond the point of bifurcation of the 

 axis. This, which is a constant and very peculiar feature, may be 

 analogous to the conjoining of the branches of Didymograpsus Moffat- 

 ensis (Carr.) " by a slight web," or even to the central disc of Dicho- 

 grapsus ; but it seems to differ from both in the conjoining portion 

 being continuous with, and undistinguishable from, the general body 

 of the polypary. The hydrothecEe are smaller and closer together in 

 the stem than in the branches, and very nearly approach each other 

 towards their outer margin, which, in the stem, has an obtuse mucro- 

 nate extension or spine, directed outwards, or at right angles to the 



