DIPLOGRAPTUS. 271 



The two basal thecse appear to have apertural spines, and in the obverse 

 aspect, as usual, the distal thecse show a characteristic Climacograptus form, 

 but the upper edge of the excavation is abrupt even for this group, and its angle 

 sometimes appears to carry a small spine, while the apertural margins are 

 somewhat introverted. In the reverse aspect, which is less commonly met with, 

 the thecse have rather the appearance of those of an Orthograptus, but with slight 

 introversion of the apertural margin. 



Affinities. — Of all the species grouped by us in Amplexograptus, A. coslatus 

 most nearly resembles a Olimacograptus. In the form of the polypary and in the 

 characters of its thecse it approximates to Climacog. typicalis of Hall, as figured in 

 his " Graptolites of Quebec Group," 1865, p. 57, pi. A, figs. 1 — \) ; but in the absence 

 of figures or descriptions of the complete polypary it is impossible to decide if our 

 form is identical with his. It is allied to Amplexog. arctus, but is a much more 

 robust form and has remoter thecse. 



Horizon and Localities. — Llandeilo (zone of Didymog. Murchisoni). 



8. Wales .- Abereiddy Bay ; Blaen-y-delyn Quarry, Fishguard. 



Associates, etc. — Lapworth's type specimens all come from Abereiddy Bay, 

 where Amplexog. coslatus occurs associated with Didymog. Murchisoni, Ortliog. 

 calcaratus, var. priscus, and other forms. 



Collections. — Sedgwick Museum and Mr. F. R. C. Reed. 



Diplograptus (Amplexograptus) arctus, sp. no v. Plate XXXI, figs. 16 a — d. 



Polypary typically small, about 2 cm. in length, widening almost imperceptibly 



to a breadth of about 1 mm. Sicuia 1*5 mm. in length, with very long 



„ 1U „ , . , , . virgella: basal thecse with conspicuous 



Figs. 187 a — a. — Amplexograptus arctus, o ' r 



s p- uov - lateral spines unsymmetrically disposed. 



t,, 4 Thecse of the general Amplexograptus type, 



long and slender, fourteen to ten in 10 mm. ; 

 overlap slight, not exceeding one half their 

 extent. 



Description. — The polypary is characteristically 



yjry \ small, but some individuals are known which have 



^^ ,' a length of 3 cm. or more, and widen very gradu- 



& c d ally to a maximum breadth of 1*5 mm. from an 



a. Sicuia and th. l' on same slab as i n iti a l width of '8 mm. The smaller forms ill 



PI. XXXI, tig. lb a. 



b. sicuia and four earliest thecse. ibid, which the breadth barely exceeds 1 mm. distinctly 



c. Proximal end ol adult specimen, " " 



obverse aspect. Enlargement of predominate. 



part of PL XXXI, fig. 16 6. l 



a. ibid. Enlargement of part of speci- The sicuia is slender, and its apex passes so 



men on same slab as PI. XXXI, . .... . . . - I . rn , 



figs. 16 b and c insensibly into the liema that it is difficult to deter- 



