42 



usually measure about one-tenth, and the narrowest about one- 

 twentieth of an inch ; and about one-sixth of the width is occupied 

 by the common body. The cellules number from thirty to thirty- 

 four to the inch. These are the dimensions of an average adult ; 

 but the stipes vary in length from one-eighth of an inch, in very 

 young members, to nearly a foot. In large specimens the stipes 

 measure an eighth of an inch or more in width. 



Didymograptus patulus differs from D. nitidus in the form of the 

 cellules, which are curved, and in the proportion of the stipe 

 occupied by the common body being somewhat larger. 



Didymograptus extensus resembles D. tiitidus and D. patulus 

 in general form, but the stipes are narrower, being not more than 

 one-fiftieth of an inch in width at the narrowest part, and one- 

 twentieth of an inch at the widest. The cellules are short and 

 broad, numbering about twenty-four to the inch. 



Didymograptus gibberulus (Fig. lo) resembles the last three in 

 having two stipes with a single row of cellules, but differs from 

 them in general form. The stipes spring horizontally from the 

 radicle, then curve backwards, until the complete form resembles 

 the letter U inverted. The cellules are very narrow, numbering 

 about forty to the inch ; and in some cases they are furnished with 

 long spines. The stipes are unusually broad compared with their 

 length. 



The genus Didymograptus is found only in Lower Silurian strata 

 in Britain, and in strata of the same age in America, in the 

 former it occurs almost exclusively in the Skiddaw Slates and the 

 Llandeilo rocks. 



The genus Tetragraptus comprises forms consisting of four 

 monoprionidian stipes, connected by a non-celluUferous rod or 

 "funicle." They resemble two Didymograpti connected at their 

 bases, the combined radicles forming the non-celluliferous funicle 

 which connects the four stipes. 



In the species Tetragraptus bryonides (Fig. ii), the connecting 

 funicle is of variable length ; and from its centre, in some cases, 

 proceeds a minute pointed radicle. The stipes diverge at various 



