502 C. Lapworth — On Scottish Monograptidce. 



In tlie fully developed portion of tlie polypary, however, they are 

 strikingly different. In the present species each theca is a narrow 

 tube, which augments its diameter as it increases in length. For 

 about half its extent it is pressed flat against the periderm — so closely 

 that the line of separation is usually invisible, even in specimens 

 preserved with their full relief. The distal half, on the contrary, 

 springs suddenly outwards, almost at right angles with the axis 

 of the polypary, and is at once doubled back upon itself, in such a 

 manner that its free inferior margin is forced round into contact 

 with that of the attached portion. The edge of the aperture is very 

 oblique, making an angle of 150° with the axis of the polypary. 



The denticle formed by the direct edge of the aperture and the 

 curved superior margin is acute, and frequently causes the species to 

 be confounded with some varieties of M. convolutus. 



At first sight of a well-preserved fragment from the fully developed 

 portion of the polypary, it is impossible to avoid inverting it, and 

 regarding the hydrothecse as similar in their form and disposition to 

 those of Mbnogr. argutus. This view is especially fortified by the 

 peculiar and apparently unbroken curve of the outer margin of the 

 theca, and by the non -visibility of any line of separation between 

 the theca and periderm ; but that our interpretation of these appear- 

 ances is the correct one is placed beyond disj^ute, not only by the 

 relations which exist between the directioai of the free part of the 

 theca, and that of the slender proximal part of the polypary in all 

 complete normally developed examples, but also by the circumstance 

 that in all bi-lateral specimens (which are abundant in some localities), 

 the apertural portion of each theca in both polyparies is directed back- 

 wards upon their common point of orig:in in the sicula. 



The nearest ally of the species is Monogr. Beclci, from which it is 

 separated chiefly by the characters of the distal hydrothecse. 



Locality. — CoDimon in the highest beds of the Birkhill Shales at 

 Mount Benger, Eldinhope, Dobbs Linn and Glenkiln. 



30. MoNOGRAPTUs Baerandei, Suess. sp. Plate XX. Fig. 5. 



GraptoUthus Barrandei, Suess; Bohmische Graptolithen, Taf. 

 ix. figs. 12a-6. 



Polypary about six inches in length, slender and very slightly 

 curved. Hydrothecse 16 to the inch, long and linear, appressed 

 to the concave margin of the periderm for more than five-sixths 

 of their length, distal extremity reflexed, and usually forming a 

 small rounded lobe. 



The proximal portion of the polypary — which may be regarded 

 as embracing the first two inches of length — is of remarkable tenuity, 

 and the periderm is little more than a mere line throughout. The 

 curvature of the polypary is scarcely percejitible in this earlier 

 portion. The hydrothecas are narrower than in any of the other 

 forms of the group, and the reflexed fraction proportionately 

 shorter. It is never more than one-sixth of the length of the theca, 

 and occasionally less than one-eighth. The characteristic lobe-like 

 termination is formed only in the fully developed part of the 



