295 



3. M. vomerinus : thecae short tubes with abrupt sig- 

 moid curvature and some torsion of axis (Middle Valen- 

 tian to Lower Salopian; Fig. 88,/). 



4. M. priodon : thecae with apertural region more or less 

 isolated and retroverted (Middle Valentian to Upper 

 Salopian ; Fig. 87). 



5. M. lobiferus : thecae with apertural region coiled into 

 a definite lobe (Middle Valentian to Lower Salopian ; 

 Fig. 88, k). 



6. M. convolutus : thecae triangular or conical, with 

 reflexed apertural margins (Middle and Upper Valentian 

 Fig. 88, /). 



7. Usually taken as a distinct genus, Rastrites : thecae 

 more or less linear and isolate, with reflexed terminations 

 (Middle and Upper Valentian ; Fig. 88, m). 



The polypary of Monograptus is usually straight or 

 slightly curved, but increased curvature resulting finally 

 in a spiral is found in all groups except the second and 

 third, and is the invariable case in the last two. 



The name Cyrtograptus is applied to monograptids in 

 which branches grow out from certain thecae, at regular 

 or irregular intervals ; the main stipe is more or less 

 spiral. Whether this is justifiably taken as a generic 

 character may be doubted ; but the species with this 

 character all occur within a narrow time range (practi- 

 cally Lower Salopian), and although some of them are' 

 almost identical in other characters with particular species 

 of Monograptus, they are not contemporary with them. 



In addition to these well-defined families, there are a 

 number of other genera, with abundant irregular branch- 

 ing, classified loosely as " dendroid graptolites." Some 

 of these may not be graptolites at all, but plants or 

 colonial animals of some other phylum. One genus, 

 however, Dictyonema (Fig. 88, a), has thecae of simple type, 

 like those of dichograptids. Its abundant branches are 



