DICRANOG HAITI'S. 



L75 



Dicranograptus ramosus (Hall). Plate XXIV, figs. 6 a, &. 



1847. Graptolithus ramosus, Hall, Pal. New York, vol. i, p. 270, pi. lxxiii, fig. 3. 

 1851. Diplograpsus ramosus, M'Coy, Brit. Pal. Foss., p. 8. 



1865. Climacograptus (s.g. Dicranograptus') ramosus, Hall, G-rapt. of Quebec Group, pp. 15, 31, 45, 4(5, 

 and 112, pi. A, figs. 18—21. 



1866. Dicranograptus ramosus, Salter?; Mem. Geol. Survey, vol. iii, p. 530, pi. xi a, figs. 1, 1 a. 

 1868. Dicranograptus ramosus, Hall, 20th Rept. of State Cabinet, p. 218. 



Figs. Ill a and b. — Dicranograp 

 tus ramosus, Hall. 



Polypary consisting of a long biserial portion about 1*5 cm. in extent, and 

 two straight uniseral stipes 4 — 10 cm. or more in length. Thecse ten to 

 eight in 10 mm. of the general Dicellograjptus type, with free outer walls 

 curved, and apertural portion strongly introverted and introtorted. Apertures 

 opening within a small but well-defined, oblique, pouch-shaped excavation 

 which occupies one third of the breadth of the stipe. 

 Description. — The biserial portion of the polypary is usually composed of from 

 15 to 18 thecse on either side, and never exceeds 1*5 cm. in length. It increases 

 gradually in width from '7 mm. to a maximum of about 1 mm., which is attained 

 at the axil. The uniserial stipes include between their dorsal walls an angle of 

 about 30°, and have an average uniform breadth of 1*2 mm. They are typically 

 straight, but specimens are occasionally met with showing a conspicuous double 

 curvature (PI. XXIV, fig. 6 h). Should the discovery of 

 more examples prove that this latter form was a perma- 

 nent one, it would be worthy of a varietal name. 



Details regarding the structure of the proximal extre- 

 mity of the biserial portion are unfortunately obscure ; 

 the sicula has not been observed, but its apertural spine, 

 when well preserved, is conspicuous. Th. V and th. I 2 , 

 as usual, give off spines when they commence their 

 upward growth. 



The remaining thecse of the biserial portion are desti- 

 tute of visible spines, as are also those of the stipes. 

 They have an average length of 2 mm. and overlap for 

 half their extent in the uniserial stipes. Their outer 

 walls are curved, and their apertures are distinctly intro- 

 verted. 



Remarks. — The typical form of Hall's Bicranog. ramosus, which occurs so 

 abundantly in the Norman's Kill fauna, New York, is by no means the most common 

 form of this species in British rocks, in which it is found at a somewhat higher horizon 

 (Lower Hartfell Shales). The species, however, is well represent ed by the two 

 following varieties (var. spinifer and var. longicaulis), which have not hitherto 



23 



a. Specimen (nat. size) of D. 



ramosus, from Hall's typical 

 district, Norman's Kill Beds. 

 Coll. Lapworth. 



b. Enlargement of proximal por- 



tion. ( x 5) 



