PETALOGRAPTUS. 275 



by the growth of th. 1" and the subsequent thecal bases. Th. I 2 here runs along 

 the side of the sicula for a short distance before curving outward. 



The first two thecas have each a length of rather less than 2 mm., and are 

 equally curved, so that the proximal end of the polypary has a symmetrical 

 appearance and is characteristically short and blunt. The thecas developed 

 subsequently are rather less curved. The angle of inclination of the thecas is 

 approximately constant in the narrower forms at 35°, but in the wider polyparies 

 the angle becomes less towards the distal end, though it is never lower than 20°. 

 A complete septum is apparently present. 



The virgula is very conspicuous as a general rule ; it is usually prolonged 

 distally, and the virgular tube has often the appearance of a vesicle. 



Affinities. — P. palmeus has often been confused with Petalog. folium, but it may 

 be readily distinguished by: — (1) the relative length and width of the polypary 

 as a whole ; (2) the short and abruptly terminated character of the proximal end ; 

 (3) the length of the sicula relatively to that of the thecas ; (4) the relative 

 length and breadth of the thecas themselves. 



Remarks. — In the classification of the Diplograptidas (p. 221) we followed the 

 plan usually adopted by previous authors (Lapw. 1873), and regarded Petal 'og. 

 folium as the type of the sub-genus. But in reality Petalog. palmeus was the form 

 to which Suess first applied the title Petalolithus, and must therefore be taken as 

 the type. 



Horizon and Localities. — Birkhill-Gala (zone of Monog. gregarius to zone of 

 M. turriculatus) . 



S. Scotland : Dobb's Linn ; Belcraig Burn ; Garple Linn ; Sundhope-on- 

 Yarrow, etc. Lake District: Skelgill; Browgill ; Pull Beck; Kentmere ; Ashgill ; 

 Mealy Gill. Wales : Conway ; Llanystwmdwy, near Criccieth. Ireland : Coalpit 

 Bay, Donaghadee. 



Associates, etc. — P. palmeus is of common occurrence in all the zones between 

 that of M. gregarius and that of M. turriculatus inclusive. The species with 

 which it is most commonly associated are Monog. gregarius, M. fimbriatus, M. 

 triangulatus, M. Sedgwicki, Ghjptog. serratus, G. tamariscus, and Climacog. Tlughesi 

 in the Birkhill Shales, and Monog. Beclci and M. turriculatus in the Gala-Tarannon 

 beds. 



Collections. — British Museum (Natural History), Sedgwick Museum, Lapworth, 

 Fearnsides, and the Authors, etc. 



Var. latus, Barrande. Plate XXXII, figs. 2 a—f. 



1850. Graptolithus palmeus var. latus, Barrande, Giapt. de Boheme, p. 61, pi. iii, figs. 3 — 6. 

 1897. Petalograptus palmeus var. latus, Elles, Quart. Journ. Geol. Soc, vol. liii, p. 195, pi. xiv, 

 figs. 5—8. 



