204 SYSTEMATIC PALEONTOLOGY 



CLIMACOGRAPTUS BICORNIS (Hall) 

 Plate LIU, Fig. 1 



Graptolithus bicornis Hall, 1847, Pal. New York, vol. i, p. 268, pi. Ixxiii, 



figs. 2a-s. 

 Climacograptus bicornis Elles and Wood, 1906, Mon. British Grapt, pt. 5, 



pp. 193, 194, fig. 125, pi. 26, figs. 8a-f. 

 Climacograptus bicornis Ruedemann, 1908, Mem. New York State Mus., 



vol. xi, pt. 2, pp. 433-437, pi. xxviii, figs. 24-26, figs. 404, 405. 



Description. " Synrhabdosome not observed. Rhabdosome linear in 

 the middle and antisicular portions; attaining great length (10 cm. and 

 more) ; gradually widening from the sicular extremity to a width of 

 2. 6 + mm., attained in about 30 mm. and maintained close to the anti- 

 sicular extremity. The former extremity is always armed with two 

 diverging lateral spines, which grow from the first two thecae, and the 

 virgella which greatly varies in length. The sicula is 1 mm. long, its 

 greater portion embedded. The thecae number 12 in the sicular region 

 and seven in the mature part and overlap about one-third their length; 

 their proximal walls are curved, the free distal wall straight and parallel 

 to the axis of the rhabdosome. The apertural margins are horizontal, 

 the apertural excavations wide (one-third length of theca) and attaining 

 in depth one-fourth the width of the rhabdosome in mature part, and 

 one-third in the earlier part. Nemacaulus not observed." Ruedemann, 

 1908. 



The typical form of this wide-spread species has been recognized in 

 North America only in the Chazyan, but varieties or very closely related 

 species occur in younger formations of the Ordovician. In southern 

 Pennsylvania and Washington County, Maryland, the upper beds (Eden) 

 of the Martinsburg shale furnish a species of Climacograptus exhibiting 

 the diverging points at the base characteristic of C. bicornis. 



Occurrence. MARTINSBURG SHALE (Eden division). Washington 

 County, Maryland. 



Collections. Maryland Geological Survey, TJ. S. National Museum. 



