TETRAGRAPTUS. 59 



the present species. T. criicifcr may be distinguished from T. Ih-mVi l)y tlio form 

 of tlie proximal end, and tlie slightly different characters of the theca?. 



Ih>ri::(>ii (iitd LocaliJi/. — Arenig, Middle Skiddaw Slates (U})per l)eds). 



LaJiC iJistrict : Barf, near Keswick. 



AsHociales, etc. — T. crucifrr occurs associated with Az. snericus and T. 

 rpiadrihrachiatHs. 



The most characteristic specimen of this species known to us was found by 

 Postlethwaite in the SkiddaAV Slates of Barf, and is now in his collection. 



Tetragraptus Headi (Hall). Plate V, figs. :3 a, h. 



1858. GmptoUthys Headi, Hall, Geol. Survey Canada Eop., 1857, p. 127. 



18G5. Craptolithus Headi, Hall, Grapt. of Quebec Group, p. 94, pi. vi, fig. 8. 



1868. Tetra(jrapsus Headi, Nicholson, Quart. Jouru. Geol. Soc, vol. xxiv, j). 131. 



1877. GraptoUlhus (Didymograpsus) Headi, M'Coy, Prodromus Pal. Victoria, Dec. v, p. 39, pi. l. 



1898. Tetragraptus Headi, Elles, Quart. Jourii. Geol. Soc, vol. liv, p. 486, fig. 11. 



Main stipes 5 — 20 cm. in length, approximately straight and rigid ; somewhat 

 slender at origin, but increasing in width to a maximum of 3 mm. A large 

 disc generally present. Thecae nine to ten in 10 mm., inclined at 40^, 

 three to four times as long as wide, in contact for more than three quarters 

 their length. Apertnral margins normal, concave, with mucronate den- 

 ticles. 

 Dp^cription. — In the forms which have a disc the stipes are preserved, as a rule, 

 with the dorsal side uppermost, though in exceptional cases the thecae are visible. 

 In those examples, how^ever, which possess no disc, the profile view of the thecae 

 is well seen. The stipes of the first order are exceedingly short — shorter, in fact, 

 than in either of the allied species (!'. quadrihrachiafus and T. crucifer). This 



would seem to indicate that the earliest developed 

 Fig. yo a.— Tetragraptus Headi, Hall. tliccffi (til. 1^ and til. 1^) are unusually Small, and that 

 the dichotomous division of the stipes takes place 

 within them. The main stipes, on the other hand, 

 are often very long, and may indeed attain a length 

 of as much as 26 cm. 

 Proximal end, siiowin^ portion of a The disc, which sccms to be prcscut in most 



disc. North of Talfan, Whitland. • i 11 i j. • i .l 



Coll. Woodwardian Museum. c<^ses, IS nearly Square, and has more or less straight 



sides. In the largest specimens examined it Avas 

 fully 17 mm. square, but it is often incompletely preserved. It is ahvays 

 slightly extended along the stipes, notably so in 3"oung forms. In one adult 

 specimen in Postlethwaite's collection there is a very prolonged " ahdloii " of this 

 kind, but this is not so pronounced in other specimens. 



