DIMORPHOGRAPTUS. 355 



Thecas twelve to ten in 10 mm., having an average length of 2 mm. and 

 overlapping one half to two thirds of their length ; apertural margins of 

 thecae of uniserial portion characteristically rostrate; those of biserial 

 portion becoming straighter and the denticle more triangular. 

 Description,— -The polypary must often have attained a great length, judging 

 from the extent of various fragments occasionally found; long, perfect specimens 



pig. 232« and b.-Dimorphograptus tfr. snowin g the proximal end are, however, rare. 

 , s (Kurckj. Thege g}iow a maximum breadth. of 2-5 mm. in 



the biserial portion, but numerous short examples 

 occur in which a breadth of 2 mm. is not exceeded. 

 The sicula has a length of 1*7 mm., reaching to 

 above the level of the aperture of th. I 1 . The virgella 

 when preserved has a length of about 1 mm. 

 Th. I 1 originates a short distance above the aper- 

 ture of the sicula, and grows upward with a 

 a b gentle outward curve ; three thecse are usually 



a. sicula with long virgella and th. l 1 . developed before the biserial portion commences. 



Enlargement of specimen on same , „, . . , T7 -, „ ,. , T . , 



slab as pi. xxxv, fig. s o. Aljinihes. — When tully preserved, D. longissi- 



b. Eeverse aspect, showing form of t -, • , • • i -i i •, • r> n ' ,t 



theca?. Enlargement of part of pi. wms may be distinguished by its size from all other 



forms except Dimorphog. physophora, from which 

 it differs in having more closely-set thecae and in the character of the proximal 

 end. Smaller specimens have, however, a superficial resemblance to D. erectus, 

 from which it may be separated by the rapidity with which it attains its maximum 

 width, and by the rostrate apertural denticles of the uniserial thecae. 



Horizon and Localities. — Lower Birkhill Shales (zone of Ortliog. vesiculosus). 

 Stockdale Shales (zone of Dimorphog. confertus). 



Lake District : Fruid Water, Tweedsmuir, quarter of a mile E. of Tarn Hows ; 

 Keisley, E. Ridlaw. 8. Scotland : Dobb's Linn. Ireland: Little River, Pomeroy. 



Associates, etc. — Dimorphog. longissimus is commonly associated with Ortliog. 

 vesiculosus, Dimorphog. confertus, D. Sivanstoni, Glimacog. rectangular^, and Monog. 

 tenuis. It is not a very abundant form. 



Collections. — Sedgwick Museum, Geological Survey of Scotland, Marr, and 

 Elles. 



Group II. 

 Dimorphograpti in which the thecas are of the general GJyptograptus type. 



Dimorphograptus erectus, sp. nov. Plate XXXV, figs. 9 a — d. 



Polypary 1 — 2 cm. in length, biserial portion straight, with a maximum breadth 

 of 1*5 mm.; uniserial portion short, slightly curved, comprising 3—4 



47 



