PETALOGRAPTUS. 273 



Sub-genus Petalograptus, Suess. 



1851. Petalolithus, Suess, Ueber Buhmische Graptolitlien, p. 20. 

 1873. Petalograptus, Lapwortli, Geol. Mag., vol. x, p. 500. 



The sub-genus Petalograptus, as here accepted, includes all those Diplograpti 

 in which the polypary was more or less tabular in transverse section, and the thecse 

 were rounded tubes approximating in form and appearance to those of Phyllo- 

 graptus. Within these limits, the outline of the polypary varies from foliiform 

 (Petalog. folium) to wedge-shaped (Cephalog. cometa). 



It is found convenient to refer to the special group or section constituted by 

 the more wedge-like forms under Hopkinson's title of Gephalograptus, and to the 

 collective group constituted by all the other forms of the sub-genus under the 

 title of Petalograptus proper. 



The appearances presented by the thecse in this sub-genus naturally vary 

 with the general outline of the tabular polyparies. In the more leaf-like forms 

 belonging to Petalograptus proper, the axis of the theca is curved and directed 

 outward, so that the apertural edge, though straight and normal, appears concave 

 and lies obliquely with respect to the general ventral margin of the polypary. 

 In the more wedge-like forms grouped under Gephalograptus the axis of the theca 

 is straight and directed upward, so that the flattened-out apertural edge is straight 

 and practically horizontal. 



The mode of development of the initial parts of the polypary is the same as 

 that characteristic of the Diplograptidae in general. But in the group Gephalo- 

 graptus we find a special modification, which consists in the postponement of the 

 growth of th. I 2 from th. I 1 until after the latter has grown up to, or beyond, 

 the apex of the sicula, and the sicula itself is thus left entirely free on one side, 

 in the unprotected manner of that in the families of the Dimorphograptidse and 

 the Monograptidse. The Gephalograpti are, however, none the less clearly true 

 Diplograptidae ; for their polypary is biserial throughout, whereas in the 

 Dimorphograptidfe the polypary is uniserial in its earlier portion and biserial 

 in its later portion, and in the Monograptidse the polypary is uniserial from its 

 commencement. 



Gkoup I. Petalograptus (proper). 



Petalograptl in which the polypary is foliiform ; proximal end somewhat pro- 

 tracted, never rounded. Sicula embedded, completely visible only in the obverse 

 aspect of the polypary. Septum complete or partial. Thecos of various lengths, 

 axis curved, apertural margins concave and oblique in compressed examples. 



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