392 



BRITISH GRAPTOLITES. 



FiGb. 260 a — c — Monograplus colonus 

 (Barrande). 





1899. Monoriraptits colonns, Periier, Etudes sur les Graptolites de Boheuie, pt. iii, sect, h, p. 9, pi. 

 xiv, figs. 3, 12, 17, text- fi-. 12. 



1900. Mo)iograj)tus colouus, Wood, Quart. Jouru. Geol. Soc, vol. Ivi, p. 463, pi. x.xv, figs. 10 a— d, 

 text-fig. 14. 



Polypary 4 — 6 cm. in lengtli, distallj straight, but with distinct ventral 

 curvature proxinially, widening gradually till the maximum breadth of 2*3 

 mm. is attained. Thecse twelve to ten in 10 mm., those of the proximal 

 portion with ogee curvature of Avails and retroverted apertural extremities ; 

 becoming simpler distally, and finally tubes of uniform width with a length 

 of 3 mm., at least four times as long as wide, overlapping fully tAA'o-thirds 

 of their length, and Avith an average inclination of 35° to 45°. Virgula 

 occasionally prolonged distally. 

 Description. — The polypary has a characteristic form, the iuAvard curvature of 

 the proximal portion in\^olving the first seven to ten thecas. The increase in 



breadth is someAvhat rapid at first and is then 

 gradual until the maximum breadth is attained ; 

 this is then maintained as a rule to the distal extre- 

 mity. The virgula may occasionally be obserA^ed 

 extending for a short distance beyond the distal 

 extremity of the polypary. 



The sicula, though small (not exceeding I'G nnn. 

 in length), is usually conspicuous; it reaches to 

 the level of the apertural margin of the second 

 theca. 



The four proximal theciie slioAV the ogee curva- 

 ture and the retroverted apertural extremities, but 

 these features are most pronounced in th. 1 and 

 til. 2, Avhicli are shorter and relatively much broader 

 than any subsequently developed. The mature 

 theca3 are very similar to those characteristic of the 

 preceding group ; they have an approximately uni- 

 form breadth, straight Avails, and an even apertural 

 margin usually devoid of a fiange ; the thecal length usually increases until it is 

 fully four times the l)readth, and may be more. 



Specimens from different British localities are apt to exhibit slight variations 

 amongst themselves ; these, hoAvever, appear to have only a local significance. The 

 specimens from Helm Knot seem to come closest to the accepted Bohemian types; 

 others from Adferton (LudloAV District) appear to Aviden rather more rapidly and 

 have mature tliecse Avliich are fully five times as long as broad; some from the 

 Builth district resemble the Bohemian forms very closely, Avhilst others show 

 more conspicuous proximal cui'vature and rather more closely set theca3 (thirteen 



a. Complete specimen preserved in re- 

 lict, natural size. Fi<:fiired, Quart. 

 Journ. Geol. Soc, vol. Ivi, pi. xxv, 

 fig. 10 A. Biitowitz, lioheiuia. 

 Coll. Laj)worth. 



h. Proximal end, showing sicula. 

 Figured, Quart. Journ. Geol. Soc, 

 vol. Ivi, p. 464, fig. 11«. Vicarage 

 Koad, Builth ; Lower Ludlow Shales. 

 Coll. Wood. 



c. Distal thecte in full relief. Vicarage 

 Road, Builth ; Lower Ludlow Shales. 

 Coll. AVood. 



