26 BRITISH CxRAPTOLITES. 



The general characters of the thecae are pecuHar ; the form of the apertnral 

 margin is highly characteristic. It is deeply concave at its inner side, but near the 



outer truncate edge it becomes slightly convex ; 



Figs. 15 5 and c. — Didyinograptus t i • '-i • •^- i 



acutidens, Lapworth MS. the apertures are nearly perpendicular m the nntial 



'' region of the stipes, but are more oblique in the 



distal parts, and the apertural angle may be as low- 

 as 70°. In the distal portions of the stipes the 

 thecae number eleven to thirteen in 10 mm., l)ut 

 they are more remote near the sicula and each 

 fc. Distal tbecse siiowing oi.iiquity of theca attains a length of 1'2 mm. The thecae 



apeitm-e. Enlaro'eiiieut of part , . ., , n ,i • t i j. j.i • 



of PI II fi"- 'od hardly overlap at all near the sicula, but this 



'■ '^hting'ia^g/lniounrif^oti Overlap increases toward the distal regions of the 



lap of the thecse and growth- gtipes to rather more than half their entire length, 



lines. Hope Stream, S. Shrop- ^ _ "^ 



shire. Coll. H.M. Geological and a vertical line drawai through one thecal aper- 

 ture cuts three different thecae. The angle of 

 inclination varies from 15" to 17°, but is rather smaller near the sicula. 



Affinities. — D. acutidens is allied to B. affiuu in the uniformly low inclination 

 of the thecse, but differs from that species in the form of the aperture and the 

 number of thecae in a given unit of length. It was included by Hopkinson in 

 D. affinis (Nich.) (' Quart. Journ. Geol. Soc.,' vol. xxxi, pi. xxxiii, figs. 6 h, r), 

 though he recognised that it differed in certain particulars from the typical form. 

 One of the specimens figured by him was found by us to be capable of further 

 development, and the removal of a small piece of the rock matrix revealed the 

 sicula and part of the second stipe, both exhibiting the characteristics of the 

 present species. The number of thecae in a given unit of length links this species 

 with JD. Nicholsoni, but the much lower inclination of the thecae and the character 

 of their apertures serve to distinguish it. In general form the polypary differs 

 from that of either of its closest allies, and approaches that of D. enodus, fi-om 

 which, however, it is readily distinguished by the characters of the thec?e. 



Hui'izon and Localities. — Upper Arenig (Llanvirn). 



St. David's District : Porth-hayog, Ramsey Island. S. Shropshire : Hope Stream, 

 a little west of Hope Quarry ; Upper Grimmer Farm ; Stream, quarter of a mile 

 north-east of Knot Moor. 



Associates, etc. — The species occurs fairly abundantly in the Lower LlanAarn 

 beds of the St. David's district, associated with J), hijidns, D. Xicholsoui, and 

 D. nanus. Several specimens are known in the collections of Lapworth, the 

 Geological Survey, the Woodwardian Museum, and the Authors. We regard as 

 type specimens those figured in PI. II, figs. 3 a and o A. The former is in tlie 

 Woodwardian Museum, and the latter in the collection of the Geological Survey. 



