C. Lapworth — On Scottish Monograptldce. 315 



generality of cases, however, the distal portion is broadly arcuate, 

 and the proximal extreraity is very sharply recurved ; the general 

 figure is consequently somewhat falciform or flagelliform {^M. 

 argutus). The peculiar curvature takes effect either dorsally or 

 ventrally — the hydrothecse lying sometimes upon the inner side of 

 the curve (Jf. Nilssoni, Barr.), and sometimes upon its outer aspect 

 {If. intermedius, Carr.). This feature remains constant for the same 

 species in all undistorted specimens, and is of great value in the 

 discrimination of allied forms. 



In the present state of our knowledge the chief distinctions 

 among the species of Monograptus appear to be : — (1) the form, (2) 

 ornament, and (3) amount of overlap of the thecse ; and (4) the 

 size and (5) shape of the complete adult polyparj^ Though it 

 would be unwise to lay down any rule in the matter, it is not 

 improbable, that a clear distinction in any one of these characters 

 ought to be regarded as entitling its possessor to specific rank. 



In endeavouring to separate closely allied species, it should never 

 be forgotten, that in all the Monograpti hitherto examined, the 

 section of the polypary formed by each hydrotheca and its attendant 

 portion of the periderm, is almost complete in all its features before 

 the hydrotheca next in order is developed. There is some evidence 

 of a slight addition to the length of the thecse ; but the remaining 

 characters are invariable. 



Almost all the errors in identification have arisen from the habit 

 of describing and figuring imperfect and fragmentary specimens. 

 Eecent research has shown conclusively that fragments from opposite 

 ends of the same polypary are so distinct in their characters that no 

 palaeontologist would think of uniting them under the same name. 

 In future a description of the complete polypary should be regarded 

 as absolutely essential to a specific diagnosis. 



Group 1. — Type Monograptus Nilssoni, Barr. 



Polypary slender and curved ; hearing long linear liydrotliecce, wMcli 



are closely adpressed to the periderm and loithout overlap. 



5. Monograptus Nilssoni, Barr. Plate X. Figs. 7 a — c. 



Graptolitlius Nilssoni, Barrande ; Grapt. de Boheme, plate ii. 

 fig. 16. 



Polypary several inches in length, slightly curved, with a normal 

 diameter of one-twentieth of an inch, and having a slowly tapering 

 proximal portion. Hydrothecse linear, arranged on the concave 

 aspect of the polypary, adpressed to the periderm, in contact only ; 

 20 to 22 to the inch ; outer margin direct, apertural margin straight, 

 and perpendicular to the general axis of the polypary. 



The longest specimen seen has a length of about six inches. The 

 adult portion maintains a constant width of one-twentieth of an 

 inch, inclusive of the projection of the hydrothecse, the apertural 

 and outer margins of which are both straight; the denticle is conse- 

 quently broadly triangular. 



The distinctive characters of this species are : (1) the concave 

 curvature and the narrow and invariable diameter of the adult 



