DICRANOGRAPTUS. 167 



Dicranograptus tardiusculus, Lapworth, MS. Plate XXIV, figs. 2 a—b. 



1882. Dicranograptus tardiusculus,~La,j)worth., Quart. Juuru. Geol. Soc, vol. xxxviii, p. 586. 



Polypary consisting of a rather long biserial portion, 8 — 9 mm. in length, and t wo 

 approximately straight uniserial stipes 1 — 3 cm. in length and of uniform 

 width ; axillary angle about 35°. Theca) twelve to ten in 10 mm., having free 

 outer Avail slightly curved, and horizontal apertures opening within wide and 

 deep excavations which occupy nearly two thirds of the width of the stipe. 

 Description. — The biserial portion of the polypary is generally composed of 

 about ten thecse on each side, and it widens very markedly throughout its length, 

 measuring only '6 mm. at its origin, but from 1'5 — 2 mm. at the axil. The uni- 

 serial stipes have a uniform breadth of about "8 mm. and are typically straight, but 

 occasionally exhibit slight curvature. They diverge at an angle of about 35°. 

 Unlike that of Dicranog. Glingani, the axil is somewhat rounded. 



The sicula is obscure, but it appears to be short and 



Fig.104. — Dicranograptustardius- , , . n -ir\A\ -j.i i j/i n i mi 



cuius, Lap-w. broad (see tig. 1<»4), with a length of only '0 mm. the 



q virgella is slender, but is usually visible. 



The. thecas are very markedly alternate in their 

 arrangement. There is distinct evidence of a septum 

 for part, if not for the whole of the length of the biserial 

 portion. Spines are commonly present as outgrowths 



ffrom the free part of the outer thecal Avail on all the 

 theeie of the biserial portion, and occasionally on the 

 t -\J( most proximal thecse of the uniserial stipes. The thecas 



ty have an average length of about 1*5 mm. and overlap for 



<[? about one third of their length. 



Biserial potion in full relief, Affinities.— D. tardiusculus has, perhaps, some slight 



showing sicula (y) and septum. M ' i ° 



Ardmiiian Brae, Baiciatehie resemblance in general form to both J), rectus and /'. 



Beds. Coll. Mrs. Gray. & 



Nieholsom. From 1). rectus it should be readily dis- 

 tinguished by the greater length of its biserial portion and by the characters of 

 the thecas, and from 1). Nicholsoni, which it resembles in the length of the biserial 

 portion, by its smaller and more slender polypary and also by the characters of the 

 thecas. The unusually Avide and deep apertural excavations are also distinctive 

 characteristics. 



Horizon and Localities. — Highest beds of Glenkiln or Lowest Ilartfell 

 (Baiciatehie Beds). 



S. Scotland. — Baiciatehie Bridge ; Ardmiiian Brae ; Benan Burn. 



Associates, etc. — D. tardiusculus, up to the present, has only been recognised in 

 Britain from the Baiciatehie Beds of the Girvan District, where it occurs asso- 

 ciated with Dvplog.foliaceus (auct) and Cryptog.tricornis, etc. The best specimens 



22 



