DIDYMOGRAPTUS. 53 



exliibits a certain amount of variation as regards the direction of the stipes. Three 

 mutations may be said to be inchided in the species : 



(1) a form in whicli the dorsal walls of the stipes are continuously recurved, so 

 that the distal extremities point towards each other ; 



(2) a form in which the dorsal walls of the stipes are straight distally, the 

 stipes running parallel to each other (PL II, fig. 9 l>) ; 



(3) a form in which the stipes diverge from each other throughout (PI. II, 



fig. 0.0- 



In all these forms, however, details of structure are essentially the same, and 

 there appears to be every gradation between the extremes. 



When the polypary is ^^ewed in the obvcrsp aspect the sicula is seen to be long 

 and narrow for the greater part of its length, but it widens somewhat abruptly near 

 the aperture, where it attains a breadth of "0 mm. In length it commonly 

 measures 3"2 mm., and a filamentous thread (nema) is often seen proceeding fi-om its 

 apex. The earliest theca originates from the sicula at a point very near the apex, 

 and then grows doAvnward ; in general form it resembles the sicula very closely 

 indeed : thus the species well illustrates the opinion held l)y some palaeontologists 

 that the body designated the sicula should be regarded as the earliest theca 

 developed from the " zooid germ." The first theca is in contact with the sicula for 

 the greater part of its length, but the two curve away from each other near 

 their apertures, leaving a space between them. Transverse lines of growth may 

 be detected along their length, and especially near their apertures. The crossing- 

 canal must be situated very high up, for the theca which is developed second is 



nearly as long as the first (cf. reverse aspect). The 

 ^"'•1^^,L;:Si,N];b';:£:''™"'^^ average length of the earlier formed thec.^ is 



about 2 mm. ; those developed later gradually 

 diminish in length towards the distal extremities of 

 the stipes, Avhich thus come to have the appearance 

 of being gently rounded off. 



In the reverse aspect of the polypary the greater 

 part of the sicula and the first theca are hidden by the 

 growth of th. 1- and tli. 2\ so that there are visible 

 p • 1 , , ^. rn o^^ly ^^16 apex of the sicula with the filamentous 



«. Proximalend, obverse view. Type . ^^<-^~> 



specimen figured by Nicholson, virgula ; the initial part of th. 1' ; and at the base the 



Aim. Mag. Nat. Hist. [4], vol. \, „ ^i . , i , , , rn,, 



xvi,i875, pi. vii, fig. 3«. Randal apertures ot the sicula and th. V. The crossnig 

 Jwun..^'""- ^^««^^^^^-^i^^° canal is clearly seen just below the apex of the 



'• "'rfgul-id b;"Elles: Quan. Jot^: ^'""''^^^ ^^^^ ^'^^^^^ ^^^^^ ^'^^ "^^^^^1 P^^^S of the 



Geol.Soc., voi.iiv, p. 499, fig. 18. tliccse wliicli have developed later. There apt)ears 



White House Fell. Coll. Wood- i , i, • i / 



wavdian Museum. also to be a crossuig caual (or something of a similar 



natiuT) between th. 2^ and th. 1'. Thus, as Torn- 

 quist points out (loc. cit.), " the first stipe crosses the sicula and the second stipe the 



8 



