360 BRITISH GRAPTOLITES. 



theca"' upon the outer side [Mouoij. ('iiplLiis) ; or in a few cases (o) dDrsi-vcuIra], the 

 curvature cliangiug sigu along the length of the })olypary {]\[oiiO(j. Jlexill.s). 



In some species the curv^ature is .sli)j}il throughout [Moiiotj. Jfexuosus). As a 

 genei'al rule, however, the curvature is distinctly 'pvonoiDict'il , but in varying 

 degrees, the complete polypary becoming arc-shaped, sickle-shaped, hook-shaped, 

 or whip-shaped in general form, the earlier portion of the polypary being almost 

 invariably more sharply curved than the mature })ortion. In a few species the 

 curvature is exresf^ire and continuous, the polypary being coiled into a plane spiral 

 (Mou.O(j. con vol lit II s^, or even into a conical helix {Monoij. tni'riculatns). 



The thecas in Monograptns differ chiefly in (1) general form, (2) amount of 

 overlap or isolation, (3) form of the apertural margin. As regards general 

 form they may vary from a simple parallel-sided tube to a tube with decided 

 sigmoid curvature, or from a tube Avith ogee-curvature to a short truncated cone, 

 these variations being often closely connected with the amount of overlap or 

 isolation. There appear to be tliecfe showing every condition from those which 

 overlap for the greater part of their length [Monocj. leptotlieca) through those with 

 partial isolation {i.e. of the apertural region only) [Moiioij. H(u]ijiricl-ii) into those 

 in which the isolation is more complete and the overlap is reduced to a minimum 

 [Monocj. convolutns). One of the characteristic features of the Monograptidas is tliis 

 tendency, in many of its species, to isolation of the thecre, the various stages of 

 which are well displayed within the genus Mouo/jraptns itself. 



There is, however, another tendency which affects the form and direction of 

 the apertural margin and also of the apertural region of the theca. In many 

 species the theca has a simple and normal apertural margin, but in others it shows 

 a certain amount of extroversion and retroversion, originating a minute tag (or 

 rostel) to the outer angle {Monoj. revolatus). In others the apertural region of 

 the theca itself becomes extroverted and retroverted {Mono(j. priodon, etc.), and 

 eventuall}^ a definite coiled-up lobe is formed {Monoij. lohifenis). The tendency 

 to isolation may sometimes act alone [Baatrltes peregriiius), but it is more generally 

 accompanied by the allied tendency to lobation. 



The sicula in Monograptns varies much in size, and it is generally minute. 

 In Monog. acinaces and Monog. gregaviiis, however, it attains the greatest extent 

 known among the Graptoloidea, the sicula in these species having a length of 

 fully 4 — 5 mm. 



The virgula is, as a rule, inconspicuous, but the dorsal impression or groove 

 marking its position is often well seen in polyparies which are preserved in full 

 relief. It is occasionally prolonged beyond the distal extremity of the polypary 

 {Monog. faiians). 



As respects the classificatory grouping of the species of the Monograpt'i, the 

 forms and relations of the thecjB afford us, as hitherto, the most satisfactory 

 criteria. But here an apparent difficulty confronts us. In some of the Monograjitl 



