Mr. C. Lapworth on new British Graptolites. 153 



before the fossil reaches its normal breadth. The hyclrothecse 

 upon this younger portion agree in their general features with 

 those upon the adult parts of the polypary, except that in 

 certain views the apertural margin appears to be proportion- 

 ally wider. When fully developed they are seen to be flat- 

 tened tubes, which, in the profile view, are more than twice 

 as wide at their point of origin as at their distal termination. 

 The proximal wall of each is bent with an elegant convex 

 curve, and is in contact with the distal wall of the preceding 

 theca for about two thirds of its longitudinal extent. The 

 apertural margin is at right angles to the axis of the hydro- 

 thecse ; it is sharply denticulate at its outer extremity, but 

 never appears to be prolonged into a distinct marginal spine. 

 The cast of the polypary (fig. 6 a) is identical with that of 

 Monograptus vomerinus, Nich. In young forms it is narrower, 

 and there is a distal prolongation of the virgula, occasionally 

 equal in extent to that of the polypary itself. The test is 

 smooth and of remarkable thickness. 



This form appears to stand midway between the typical 

 form of M. galaensis, Lapw., and M. colonics, Barr. From 

 the former it is separated by the much greater size and thick- 

 ness of the polypary, the angle of inclination and amount of 

 overlap of the theca, and by the prolongation of the virgula 

 in young examples. From the latter it is distinguished by 

 the far greater length of the polypary, by the fewer hydro- 

 thecae upon the same longitudinal extent, and by the form 

 of the apertural portion of the theca and the absence of 

 ornament. 



Horizon and Locality. Common and well preserved in the 

 zones of Cyrtograptus Murchisoni, Carr., and G. Linnarssoni, 

 Lapw. (Wenlock), at Wellfield, Builth, river Onny, &c. 



6. Monograptus crenularis, sp. nov. 

 (PI. IV. figs. lOa-LOe.) 



Polypary of great length, rigid, slightly curved throughout 

 the whole of its extent ; average width one sixteenth of an 

 inch. Hydrothecse arranged on the convex margin of the 

 polypary, twenty to twenty-two to the inch, inclined at an 

 angle of about 15° to the axis of the polypary, long narrow 

 tubes, overlapping each other for less than half their length, 

 slightly diminishing in diameter in the direction of the aper- 

 ture, the margin of which is somewhat oblique, deeply con- 

 cave and inornate. 



The polypary in this species attains a total length of at 

 least eight inches. It is almost straight throughout ; the very 



