DIDYMOGRAPTUS. 



41 



Figs. 25 a, h, and c. — Didymogrui^ius Mnrchi- 

 soni, var. gemimts, Hisinger. 



Proximal end, obverse view. Gelli, Ll;in- 



drindod. Coll. Geological Society of 



London. 

 Proximal end, reverse view, on same slab as 



tyfie specimen of D. Mnrcliisoni. Gelli. 

 Proximal end, reverse view, on same slab 



as type specimen of D. farcillutus. 



Aberciddy Bay. Coll. Woodwardian 



Museum. 



and tapering; the "wliole proximal end more slender, and the base 



generally more rounded than in D. MiircJiisoni. The thecae number twelve 



in l(t mm. ; their initial angle of inclination is 45°, but rises to 55° at the 



aperture OAving to curving. They are three times as long as wide, and 



are free from a half to one third their length. 



Description. — The stipes widen abruptly from about '0 mm. to 2'5 mm. Small 



specimens about 2 cm. in length are most common, but longer forms are also 



fairly numerous. 



The sicula is about 2*6 mm. in length, 

 and has a width at its aperture of about 

 •75 nun. Th. 1^ originates slightly above 

 the base, and curves away outward and 

 downward so as to leave the sicula free on 

 the left side (obverse view) for a small 

 fraction of its length. The crossing canal 

 is slightly oblique. There is a general 

 appearance of symmetry as regards the 

 origin of the stipes. 



The earliest thecfe are short, being only 

 1 mm. and 1'5 mm. in length, but each one 

 subsequently developed shows a decided in- 

 crease in length on the preceding one, and exhibits marked curvature of its 

 walls — facts upon which tlie characteristic rapid widening of the stipes depends. 



Affinities. — D. Mnrchisoni, var. geminus, has been 

 compared by some writers in this country with 

 ]J. vanus (Lapw.). These two forms should be 

 readily separated, var. (jciiiinvs differing in the 

 absence of parallelism of the stipes, and in its rapid 

 increase in width; while in D. nanus the stipes are 

 parallel and the width uniform. 



It resembles JK hifidns in its general shape, but 

 may be distinguished by the characters of the thecse, 

 and the number in a given unit of length. 



Horizon (iiul Localities. — Lower Llandeilo (zone 

 of D. Mnrchisoni). Radnorsliire : Gelli, Jjlandrindod. Shropsliirc : Holywell 

 Burn. Fembrokeshire : Abereiddy Bay. 



Associates, etc. — D. MnrcJiisoni, var. r/eminns, occurs abundantly in the Lower 

 Jilandeilo beds, associated Avith the typical form, and with species usuall}' referred 

 to JJipJof/rajjtits foliaceus and Cryptograptus tricornis. Specimens are in the 

 collections of the Geological Societ}^ the British Museum (Natural History), the 

 AVoodwardian Museum, Liqjwortli aud the Authors. 



6 



Fig. 25 d. — DUhjmogya2)tiis Mmxld- 

 soni, var. geminus, Hisinger. 



Proximal end in sb:ile, reverse 

 view. Zone of D. geminus. 

 FSgelsang, Sweden, 

 qvxist, Lund. 



Coll. T5rn- 



