MONCXIRAPTUS. 361 



tlie tliecas are of tlie same type [iniifonn) tlirougliout tlie -whole length of the 

 polypary ; in others two types at least occur {hi-form), one in the earlier portion 

 and another in the later or mature portion. In the families of the Dichograptida?, 

 Dicranograptidge and Dimorphograptidse also the earlier thecffi occasionally differ 

 from those subsequently developed in the later or more mature stages of the 

 growth of the polypary, but the latter have been ahvays considered to be the more 

 characteristic and distinctive. There can be little doubt, therefore, that in the 

 matter of the classification of the species in the Moiiofpufpii the characters of the 

 thee* in the mature portion of the polypary should similarly take systematic 

 precedence of those of the earlier portion. On this basis the Monorirapti fall into 

 seven main groups, which can be further subdivided. 



Group I. — Munograjitl in Avhich the mature thecal are simple, straight, over- 

 lapping tubes with approximately parallel walls and even apertural margins. 

 Type Monoij. cj/plms. 

 GrROUP II. — Moiior/rapti in which the mature thecEe are tubes with flowing 

 sigmoid curvature and oblique apei'tural margins. 

 Tjqje Moiiog. Sandersoni. 

 (iiv'oup III. — Monograpti in which the mature theca? are short tubes with abrupt 

 sigmoid curvature and some torsion of the thecal axes, so that the apertural margins 

 show occasionally within distinct excavations in the ventral margin. 

 Type Monog. vovwriniis. 

 Group IV. — Monogmptl in which the mature thecas are tubes Avith the apertural 

 region more or less isolate and retroverted. 

 Type Movog. prioJon. 

 Gkoup V. — Monograpti in Avhich the mature theca3 are tubes with the whole 

 apertural region coiled into a definite lobe. 

 Type Moiiog. lohifcrns. 

 Gi;oup VI. — Monograpti in which the mature tliecfe are triangular or conical in 

 form with reflexed apertural margins. 

 Type Moiiog. conrulntiis. 

 Group VII. — Monograpti in which the mature theca? are more or less linear and 

 isolate Avith reflexed terminations. 

 Type liast rites pvrcgrinus. 



Group I. 



Monograpti in which the mature thecfe are simple, straight, overlappino- tubes 

 with approximately parallel walls and even apertural margins. 



The Graptolites included in this group may be further divided into sections 

 and sub-sections according to Avhether the thecre are uniform or biforrn, and 

 according to the kind of curvature of the poly})ary. 



