THE GRAPTOLITES AND CORALS 291 



Branching usually dichotomous, but occasionally ir- 

 regular. Primary angle of divergence generally 180 or 

 less. Tremadocian to Middle Llandeilian. 



Chief genera are those mentioned on Fig. 85. Of 

 these only Didymograptus ranges above the Lower 

 Llanvirnian (bifidus zone). 



Family LEPTOGRAPTID^E. Uniserial, with slender 

 flexuous polyparies ; primary angle of divergence ap- 

 proximately 1 80; branching usually lateral; thecae 

 elongated, with slight sigmoid curvature, apertures in- 

 clined, situated partly within depressions (excavations), 

 somewhat introverted (turned in wards), but not introtorted 

 (twisted inwards). Range in Europe : Middle Llanvir- 

 nian to Middle Ashgillian, but in Australia a species of 

 Leptograptus is associated with other graptolites of Skid- 

 da vian facies. 



Chief genera: Leptograptus (Fig. 88, b), without lateral 

 branching (range of family) ; Nemagraptus (Fig. 88, c), 

 S-shaped polypary with numerous lateral branches 

 (Middle Llandeilian); Pleurograptus, with compound lateral 

 branches (one zone in Hartfell Shales, Ashgillian). 



Family DICRANOGRAPTID^;. Uni-or uni-biserial, angle 

 of divergence always exceeding 180; thecae with strong 

 sigmoid curvature; apertures horizontal or inclined, 

 situated within well-defined "excavations" and fre- 

 quently introverted and introtorted. Llanvirnian to 

 Ashgillian. 



There are two genera : Dicellograptus (Fig. 88, d), 

 in which the two branches rise up somewhat as 

 in the reclined dichograptids, and Dicranograptus 

 (Fig. 88, e), in which they are united at first (as in 

 Phyllograptus or Climacogmptus), but afterwards separate. 

 This family appears to be transitional between Lepto- 

 graptidae and Diplograptidae, but Dicranograptus in 

 its life-history shows apparently catagenesis from the 



