IMIOC'TIOCJRAPTUS. 73 



Genus TROCHOGRAPTUS, Ifulm. 

 1881. Trochngraptn^, Holm, Ofv. Kongl. Vet. Akad. Siockliolm Fiirb., vol. xxxviii, No. 9, p. 49. 



Poll/par 1/ robust, typically bilaterally symmetrical, consisting of four main 

 stipes, several centimetres in length, produced by dicliotomous division 

 from a primitive Didijmograptus stage. From each of these four main 

 stipes numerous compound lateral branches several centimetres in length 

 are given off at irregular intervals. 

 Thecx narrow tubes of the ordinary Dichograptid type. 

 The polypary is characterised by the possession of stipes of two orders only ; 

 but from the stipes of the second order (main stipes) compound lateral branches are 

 given off. It may therefore be regarded as a Tetragraptuii of the quadrihracMatus 

 type, bearing compound lateral branches on its main stipes. It differs from Scliizo- 

 graptiis in having compound instead of single lateral branches, and in having them 

 disposed at irregular instead of regular intervals. 



The Trochograpti often attain a very large size, and, as a general rule, there is 

 no difference in the width of the main and lateral stipes. The distance between 

 the lateral branches increases usually with remoteness from the sicula, and the 

 angle at which they are given off usually decreases with the complexity of the 

 branch ; this is, however, not invariably the case. The thecce are as a rule obscure 

 on the main stipes, but can often be fairly well seen on some part of the lateral 

 branches. 



Only one species has as yet been described, viz. T. diffnsus, Holm. 



Trochograptus diffusus, Holm. Plates VII and VIII, figs. 1 a, b. 



1881. Trochograptus diffusus, Holm, Ofv. Kougl. Vet. Akad. Stockholm Forli., vol. xxxviii, No. 9, 



p. 49, pi. xiii. 

 1898. Trochograptus diff'usus, Elles, Quart. Journ. Geol. Soc, vol. liv, p. 479, pi. xxvii. 



Main stipes and compound lateral branches curved, often several centimetres in 

 length, robust, having an average width of 2 mm. ; lateral branches always 

 situated on same side of main stipe, and curved in the same direction; 

 distance between lateral branches commonly increasing with remoteness from 

 the sicula, while the angle of divergence decreases with the complexity of 

 the stipe. Thecoe eleven in 10 mm., inclined 45°, average length 3 mm., six 

 times as long as wide, outer wall straight, overlapping three quarters their 

 length. Apertural margin concave, normal. 



