C. Lapioortli — On Scottish Monograptidw. 547 



latter view of their origin, though extremely natural, has always 

 had two difficulties in the way of its acceptance ; viz. the distinct 

 isolation of the sicula in those genera, and the fact that they 

 appear all to have become extinct long prior to the advent of 

 the first of the Mbnograptidcd. In the Diplograptidcd, on the other 

 hand, the sicula is adherent, and individuals swarm abundantly in 

 the same beds with the Monograptidcs. 



On this view of the relationship of the two families, we may set 

 down the coalescence of the branches in DimorpJiograptus to partial 

 reversion. To the same cause may perhaps be ascribed the bilateral 

 character of the polypary in the abnormal examples of Monograptiis 

 runcinatus, etc. 



37. DiMORPHOGRAPTUS ELONGATUS, sp. nov. Plate XX. Figs. 12a-&. 



Polypary one and a half to two inches in length ; proximal portion 

 monoprionidian, slightly recurved, slender and tapering; distal 

 portion diprionidian, expanding to a maximum width of one- 

 twelfth of an inch. Virgula distally prolonged. Hydrothecee 20 

 to the inch ; making a small angle with the axis of the polypary ; 

 long, slender, without overlap ; apertural margin of each much 

 inclined ; denticle acute, conspicuous. 



The sicula, which is slender and about one-tenth of an inch in 

 length, lies upon the dorsal aspect of the proximal extremity of the 

 polypary, as in Monograptus. In some examples there is a short 

 extension of the virgula, directed proximally in the line of the 

 suture between the sicula and polypary. The proximal mono- 

 prionidian portion of the polypary has the peculiar dorsal curvature 

 so frequently seen in Monograptus. The earlier hydro thec^ are 

 pressed flat against the periderm ; the apertural margin of each 

 transgresses for some distance upon the surface of the one which 

 precedes it, and forms a very minute excavation. The latter is 

 shallow, and becomes less and less appreciable as the polypary 

 increases in length. The thecse themselves are of the general type 

 of those of Monogr. tenuis, Portlock, and of those of the Diplograpii 

 resembling Diplogr. tamariscus, Nich. In the distal portion of the 

 polypary the shallow apertural margin of each, which has previously 

 stood at right angles with the general ventral edge, gradually 

 increases its inclination till finally it makes an angle of about 45° 

 with the axis of the theca, and the denticle becomes long and very 

 acute. This part of the fcssil, when found in fragments, is un- 

 distinguishable from the corresponding portion of Diplograptus 

 acuminatus, Nich., which occurs in abundance upon the same horizon. 

 Fragments occur in association with this species which seem to be 

 wholly monoprionidian. They possibly belong to this form ; as in 

 South Scotland no true examples of Monograptus have yet been met 

 with so low in the Moffat Series ; but our knowledge of the species 

 is as yet too limited to enable us to determine the point with 

 certainty. 



Locality. — DimorpJiograptus elongatus is a Nery rare fossil in the 

 lowest zone of the Birkhill Shales at Dobbs Linn and Craigmichan. 

 1 See Geol. Mag. for Nov. p. 507. 



