C. Lapivorth — On Scottish MonograptidcB. 351 



This is the only Scottish form that can be referred to the Prionotus 

 Sagittarius of Hisinger (Lethea Suecica, pi. 38, fig. 7). It agrees 

 with his species in the convexity of the outer edge of the theca, and 

 in the concavity and obliquity of the apertural margin. (Comp. 

 Plate XII. Fig. 1 e.) 



Locality. — Eare in the Gala Group at Grieston, Broadmeadows, 

 Glendearg, Packman's Barn, etc., and in the Eiccarton Beds at Eicoar- 

 ton, Edgerston, etc. 



Var. (a.) jaculum, Lapw. Plate XII. Figs. 2 a-d. 



Polypary similar in form to that of M. Hisingeri. Hydrothecae 

 overlapping each other for half their length, inclined at an angle of 

 from 30° to S5° ; narrow, with direct margins and expanded aperture, 

 the edge of which is at right angles with the virgula, and originates 

 a conspicuous acutangular denticle. 



In the great majority of instances the polypary in this form 

 appears to be quite straight throughout the whole of its length, which 

 frequently exceeds ten inches. There is, in reality, however, a slight 

 dorsal curvature, just sufficient to enable us to recognize the fact 

 that the hydrothecse are placed upon the concave margin. 



The hydrothecse are conical tubes, compressed below and having 

 an expanded orifice, the edge of which, instead of being placed at 

 right angles to the general direction of the theca, as in most Mono- 

 grapti, is perpendicular to the virgula. The outer edge of the 

 theca being straight, the denticle is necessarily acutangular in form, 

 and is remarkably conspicuous. In specimens preserved in half- 

 relief, the thecae are seen to overlap one another for half their length 

 — the point of origin of each lying precisely opposite the centre of 

 the theca immediately preceding. In casts of the interior in iron 

 pyrites, the groove in the cast formed by the separating walls is 

 carried outwards to the virgula, and we have the fallacious appearance 

 of even a far larger amount of overlap. 



The most common appearance presented by this species is that 

 given in Fig. 2c, the hydrothecse being compressed in such a manner 

 that the apertural margin of each stretches half-way across the body 

 of the periderm. 



Locality. — Confined to the highest zone of the Birkhill Shales at 

 Dobbs Linn, Garple, Belcraig, etc. 



With rare exceptions, all the members of the family of the 

 DichograptidoB possess hydrothecse which appear after compression 

 identical with those of M. Eisingeri. liy a very natural error all 

 fragmentary branches of species of that family have consequently 

 been hitherto I'eferred to this form under its commoner title of 

 Graptolites Sagittarius. Owing to the objectionable custom of deter- 

 mining species from fragments alone, the farther mistake has been 

 committed of confounding all the Monograpti of the present group 

 under the same title. The perplexing confusion which has been the 

 inevitable result of these erroneous determinations has effectually 

 prevented the introduction of correct views regarding the vertical 

 distribution of the various Graptolitic species. 



