500 ^ C. Lapicorth — On Scottish Monograptidce. 



of the theca itself. The terminal portion of the latter is bent 

 abruptly backwards in the direction of the proximal extremity of 

 the pol3^pary, its inferior margin pressing against the central por- 

 tion of the theca, or even slightly transgressing upon its surface. 

 This is the origin of the remarkable thecal lobe which constitutes 

 the chief characteristic of the whole group. The external orifice is 

 brought round in this way so as to open in the direction of a point 

 on the ventral margin of the polypary, lying a little on the proximal 

 or first-formed side of the base of its own hydrotheca. The apertural 

 margin is quite straight, and is usually wholly devoid of ornament. 



The number of hydrothecas to the inch is far from constant, and 

 there are at least three prime distinctions in their shape and propor- 

 tion. In what may be regarded as the typical form of the species (Figs. 

 1 a, h), they undergo scarcely any contraction distally, and the lobe is 

 consequently large and ball-like. In the commonest variety (M. 

 Nicoli, Hark. ?) they are short and pointed, and specimens occasionally 

 occur almost inseparable from extreme forms of M. convolutus {com- 

 mimis) ; while in a second variety ( = millepedia, M-*Coy) they are 

 long and tubular, and the lobe is scarcely distinguishable. 



Locality. — Very common in the middle and higher beds of the 

 Birkhill Shales at Belcraig, Glenkiln, Garple, Dobbs Linn, Craigie- 

 rig, etc. 



27. MoNOGRAPTus Becki, Barrande, sp. Plate XX. Fig. 2. 



Graptolithus Becki, Barrande ; Grapt. de Boheme, pi. iii. figs. 



14 to 18. 

 Monograpsus BecM, Geinitz ; Die Graptolithen, Taf. iii. fig. 13. 



Polypary long and narrow, straight (?), or with a slight ventral 

 curvature. Hydrotheca 20 to 25 to the inch, free, either wholly or 

 in part, outer extremity of each forming a rounded lobe. 



In the preceding species the early hydrothecas resemble those on 

 the fully developed portion of the polypary in everything but size, 

 Here, on the contrary, they are distant, long, and linear, with their 

 apices only slightly reflexed ; while the later ones are closely set, are 

 subtriangular in form, and have nodose terminations. Again, the 

 former are pressed flat against the ventral margin, and only a very 

 small fraction of their length is isolated ; while the bases of the 

 latter are in contact, and they are free and isolated throughout the 

 whole of their extent. That portion of the polypary which carries 

 these less developed thecce is often a couple of inches in length. In 

 the Birkhill examples it forms a gentle and continuous curve, the 

 thecse being disposed upon its concave margin. In the much more 

 recent specimens afforded by the shales of the Gala Group, the first 

 half-inch is bent slightly backwards, and examples now and then 

 occur in the transition beds which are almost straight. 



In this species we very frequently meet with those examples in which 

 the sioula, at the same time that it throws off" the normal series of 

 hydrothecce in the usual manner from one of its lateral margins, also 

 gives origin to a second series, which is developed in a direction 

 exactly' 02:)posite. The two primary hydrothecas are symmetrically 



