352 



BRITISH GRAPTOLITES. 



Collections. — Museum of Belfast Natural History Society, Sedgwick Museum, 

 Lapwortli, and the Authors. 



Dimorphograptus decussatus, sp. nov. Plate XXXV, figs. 5 a- 



Figs. 229 a and b. — Dimorphograptus 

 decussatus, sp. nov. 





5"' 



- 



^t 



si 



Polypary 1 — 1*5 cm. or more in length, uniserial portion very short and 



slightly curved, biserial portion straight, maximum breadth 2'5 mm. 



Sicula conspicuous. Thecse fourteen in 10 mm., uniserial thecse almost 



isolated, biserial thecse of general Orthograjptus type. 



Description. — The character of the uniserial portion of the polypary is unique 



within the genus; it is extremely short, comprising 

 only 2 thecse, and all the earliest thecse are almost 

 isolated from each other, approaching in type those 

 characteristic of Monog. convolutus. In the distal 

 portion of the polypary, however, they are of the 

 type of Orthograptus and overlap about one half their 

 length in the normal manner. The walls of the 

 earlier biserial thecse are strongly curved, and are 

 inclined at a high angle, while those of the later ones 

 are straighter and inclined at a low angle. Hence 

 the maximum breadth of the polypary is attained 

 near the commencement of the biserial portion. 



The sicula has a length of 1/7 mm., but the apex 

 is often concealed by the thecal bases ; th. I 1 arises 

 slightly above its aperture, and, growing upward, 

 eventually bends outward somewhat abruptly ; th. 2 1 

 arises from th. I 1 at the bend of the latter, and 

 growing outward with strong curvature has both 

 walls completely free ; th. I 2 arises from th. 2 1 , and growing similarly in an opposite 

 direction, forms an almost isolated theca on the other side of the polypary; the 

 next two thecse also grow in the same way, and all the proximal biserial thecse are 

 markedly alternate in their arrangement. After the development of th. 3 1 and 

 th. 2' 3 a partial septum seems to make its appearance within the polypary, and the 

 thecse on each side develop individually from the one next below ; then their walls 

 become straighter, and the amount of overlap increases until a constant breadth of 

 2 mm. is reached by the polypary. 



Affinities. — I), decussatus is readily distinguishable from other Dimorjphograjpti 

 by the characters of the uniserial thecse. 



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

 S. Sent I n nd : Long Linn ; Main Cliff, Branch Linn, and Corrie, Dobb's Linn. 



a. Complete specimen, reverse aspect. 



Enlargement of PL XXXV, fig. 5 c. 



b. Young specimen, obverse aspect, 



showing complete length of sicula. 

 Main Cliff, Dobb's Linn; Birkhill 

 Shales (Monog. tenuis band). Coll. 

 Elles. 



c. Young specimen, obverse aspect. 



Dobb's Linn; Birkhill Shales (zone 

 of Orthog. vesiculosus). Coll. Elles. 



