CLIMACOGRAPTUS. 



180 



Climacograptus medius, Tornquist. Plate XXVI, figs. 4 a — /. 



1870. Climacograpsus teretiusculus, Nicholson (pars.), Ann. Mag. Nat. Hist., ser. 4, vol. vi, p. 373, 

 figs, la, J,/. 



1872. Climacograpsus teretiusculus, Nicholson, Monog. Brit. Grapt., p. 33, figs. 8 a, b,f. 



1873. Climacograptus scalar is, Malaise, Terr. Silur. du centre de la Belgique, p. 104, pi. vi, figs. 5, 6. 

 1897. Climacograptus medius, Tornquist, Diplog. and Heteroprionidse of Scanian Rastrites IVils, 



Acta Eeg. Soc. Physiogr. Lund, vol. viii, p. 7, pi. i, figs. 9—15. 



Polypary 1 — 4 cm. in length, wide at origin, but undergoing slight increase 

 for about the first centimetre of its length, until a maximum width of 

 about 2*5 mm. is attained. Septum partial till the level of fourth or fifth 

 thecal pair, thence complete. Sicula 1'5 mm. in length but partly con- 

 cealed. Virgella very long, attaining a length of 10 mm. or more. Theca) 

 twelve to ten in 10 mm., of the same general tvpe as those of 01. scalaris. 

 Description. — The sicula is free for a considerable fraction of its length on one side, 

 and in addition to the virgella there occur occasionally two basal spines, the origin 

 of which is obscure ; they seem to belong to the sicula rather than to the thecae. 



In the obverse aspect the septum is seen to extend down to the proximal 

 p.os. 122 a, b, *nd c.-ciimacograptns extremity of the polypary ; in the reverse aspect, 



however, there is no trace of it until the level of 

 the fourth pair of thecae is reached, and occasionally 

 not until the level of the fifth ; this would seem to 

 imply that it is at first only partial, not extending 

 completely through the polypary until the level of 

 the fourth or fifth thecal pair. 



The virgella varies in length from 5 mm. to 

 10 mm. ; it is always therefore a more conspicuous 

 feature than in CI. scalaris or 01. recta ngularis. 

 The virgula is commonly prolonged distally for a 

 considerable extent. 



Affinities. — 01. medius is no doubt intimately 

 related to 01. rectangularis and 01. Tornquisti, and 

 may be regarded as an intermediate form between 

 them. It agrees with 01. rectangularis in point of 

 size and in the characters of the thecae; it differs, 

 however, (1) in being wide at the proximal end 

 and hence undergoing less increase to attain the 

 same breadth, (2) in the possession of a much 

 longer virgella, and (3) in the fact that the septum 

 is not completely developed till the level of the 

 fourth or fifth pair of thecae. In compressed 



25 



. Proximal end, obverse view. Main 

 Cliff, Dobb's Linn ; Birkhill Shales 

 (zone of Diplog. vesiculosus). Coll. 

 Elles. 

 Small specimen showing two spines 

 in addition to virgella. On same 

 slab as PI. XXVI, figs. 4 a, c, and e. 



Distal theca' showing large apertural 

 excavations. Enlargement of part 

 of PI. XX VI, fig. 4a. 



