DIDYMOGRAPTUS. 11 



towards their apertures, being often twice as broad there as at their origin. Their 

 outer walls show a slight sigmoid curvature; near the base they are somewhat 

 _ ^, ^., , .,., ,„ „, convex, near the aperture concave. They are in- 



FiG. 5 6. — Dulymograptusnitidus (tlaAl). '^ '' 



clined at an average angle of about 40°, but, owing 



i^^^(^><^^f^^/^^^f'<v^^^ to the curvature of the cells, the initial angle may be 



as small as 35°, while the angle near the aperture 



Distal thecae of same in i-elief. .„, . . 



may be as large as 50 . They are m contact tor 

 one half their length in the proximal part of the stipe, but this amount of overlap 

 increases usually to two thirds, and may even be as much as three fourths, near 

 the distal extremities. The apertural angle is normal. The thecae frequently 

 present a " constrictiform " appearance. 



Remarks. — /). nitidus, as here described, does not include those forms from 

 the Skiddaw Slates originally identified and described by Nicholson as Hall's D. 

 nitidus (see above), but even without these the British forms appear to exhibit a 

 greater degree of variation than those of America. Particularly is this the case with 

 regard to the angle of divergence. In Britain there are, however, a number of forms 

 which agree in all essential particulars with the description given by Hall, but these 

 graduate so insensibly into other varieties that it is not advisable to separate them. 

 „ ^ ,, ,,.j ^ .,., Affluities. — D. 7i'i^i(/;(6' in its typical form is most 



r iGS. 5 c and a. — Didymograptiis nitidus '^ ^ '■ 



(Hall)- closely allied to Hall's D. patulus ; in fact, it differs 



i^ from that species mainly in the greater number of 



^>v'"'^^ thecEe in a given unit of length. Some forms also 



c. Proximal end, obverse view. From reseiublc D. uuiformis, sp. uov., ill general appcar- 



Hall's original locality, beach below t fo ■ ^ • n • i t • i 



Pt. Levis. Coll. Lapworth. ancc, Dut ditter lu having a smaller sicula, and m the 



gradual increase in width of the stipes. 



D. nitidus also approaches D. simnlans in general 

 form, but differs in the proximal end and the 



d. Eeverse view of same. , , c ,^ J_^ 



characters ot the tnecae. 



Horizon and Localities. — Arenig, Middle Skiddaw Slates (Lake district) ; 

 Middle Arenig Hicks (St. David's) ; Mytton beds (S. Shropshire). 



Lake District : Randal Crag, Skiddaw ; Ullock Pike, below Raven Crag, ^Y. 

 Skiddaw ; east of Longside ; Bassenthwaite ; Connor ; Knott Head ; Whinlatter 

 Pass ; BrundelhoAv Lead Mine ; Gatesgill. St. David's District : Noi'th of Talfan, 

 Whitland. Lleiju Peninsula : Aberdaron. 8. Shropshire : Lady well ; Shelve. 



Associates, etc. — This species occurs in the Skiddaw Slates associated with Tetra- 

 graptus quadrihrachiatus (Hall), in the St. David's district associated with T. serra 

 and D. exteiisus, and with similar associates in S. Shropshire. Specimens from all 

 these areas are in the collection of the Woodwardian Museum, and some from the 

 Skiddaw Slates in the collection of the Keswick Museum and the private collection 

 of Mr. Postlethwaite, Keswick. 



