354 C. Lapivorth — On Scottish Monograptidce. 



species the distal moiety of each is perpendicular, while the proximal 

 portion is inclined at an angle of about 30° — its free edge being im- 

 pressed so as to form a deep excavation, into which opens the mouth 

 of the hydrotheca immediately preceding. This excavation, the 

 position and shape of the theca, and that of the thecal aperture, are 

 all identical with those in the widely-removed genus Climacograptus. 

 A further and most remarkable similarity is seen in the presence of 

 a short stiff spine, which springs from the outer margin of the theca 

 at the angle forming the distal termination of the excavation. This 

 ornament (the mesial spine) is elsewhere strictly confined to the 

 genus Climacograptus, Hall, and its most intimate allies ; and is 

 unknown among the Monograpti outside of the present species. In 

 the adult portion of the polypary this spine is almost horizontal ; 

 but in the first half-dozen hydrothecse, it is directed partly outwards, 

 and partly towards the proximal extremity. The denticle, as a rule, 

 is mucronate, and without ornament, but occasionally there is a 

 rudimentary apertural spine, much inclined. 



M. vomerinus is perhaps the best characterized species in this 

 genus, differing from all others in the form and ornamentation of 

 the hydrothecas. 



Locality. — Occurs in abundance in the Riccarton Beds at Elliotts- 

 field, Shankend, Hindhope, etc. ; and in the Lower Pentlands at 

 Habbies Howe. 



§ '^• 



Group 3.— Type, Monograptus Halli, Barr. sp. 



Polypary straight, stout, rigid; bearing wide overlapping tubular 

 hydrothecce with everted apertures, usually furnished with short 

 apertural spines. 



18. Monograptus Halli, Barr. Plate XIII. Figs. 1 a — d. 



Graptolithus Halli, Barr.; Grapt. de Boheme, pi. 2, figs. 12, 13. 

 Monograpsus Sedgwichii, Geinitz ; Die Graptolithen, Taf. iii. 



figs. 2, 3, 4. 

 Monograpsus Halli, Geinitz, ibid, figs. 5, 6, 7, 8. 

 Polypary several inches in length, straight or with a slight dorsal 

 curvature, and having an average diameter of one-tenth of an inch. 

 Hydro thecse short and broad tubes, with a subquadrate section, 18 to 

 20 to the inch, inclined at an angle of from 50° to 60°, overlapping 

 each other for about half their length, slightly contracting in the 

 direction of the aperture, the margin of which is broadly convex, 

 giving rise to a well-marked denticle, which is ornamented with a 

 stout horizontal spine. 



Commencing with a rapidly-expanding proximal portion, the 

 polypary attains its normal diameter within the first three inches 

 of its extent. Both straight and curved specimens are common ; in 

 the latter the hydrothecse are arranged upon the cc)ncave margin. 

 These afford the distinguishing characteristics of the S23ecies. In 

 specimens preserved in relief, they are seen to be subconical in form, 

 contracting slightly throughout, so that the distal portion of each 



