Development and Mode of Growth of Diplograptus. 243 



dosome, and that the virgula does not protrude beyond the 

 proximal end, unless the periderm of the rhabdosome is broken 

 away, leaving the virgula free. This, however, is different in 

 my material, which shows the virgula to extend beyond the 

 proximal point of the sicula into the hydrocaulus, and even 

 beyond the proximal end of the rnabdosome into the hydrocau- 

 lus. A very interesting specimen bearing on this question is rep- 

 resented in PL ir, fig. 6. The virgula, a shining, chitinous rod, 

 contrasts with the thinner film of the hydrocaulus. The latter 

 is broken at m- the more solid and inflexible virgula, however, 

 has separated from it and lies now partly outside of the canal, 

 still preserving its natural position in the sicula. 



Nicholson, too, claims to have seen in Diplograptus jpristis the 

 common canal " without denticles " continued on each side of the 

 prolonged rod; and Allmann states that if the virgula, as variously 

 observed, extended beyond the young growing portion of the 

 stipe, it must have been included in a coenosarc and this confined 

 in a perisarc, " which was probably still so delicate as to be 

 incapable of preservation, its thicker rod-like portion being the 

 only part preserved." As the above described specimen shows, 

 this thin perisarc, the hydrocaulus, has been preserved under 

 favorable conditions. 



Wiman further came to the conclusion that the sicula was 

 either open or had a very thin wall at the pointed end. The 

 study of my specimens of sicul^e furnished evidence that the 

 sicula was attached with this pointed end by means of u filiform 

 process either to the parent colony, or, when the sicula was free, 

 to the central node of the basal appendage. In both cases the 

 rhabdosome developed-along this hydrocaulus toward the center 

 of the colony. That might induce the notion that the hydrocau- 

 lus became the common canal of the rhabdosome. Wiman, 

 however, has demonstrated that a common canal, as progenitor 

 of the hydrotheca^, does not exist, but that the common canal 

 is the result of the growth of the thecas. The results of 

 Tornquist's* excellent researches agree with Wiman's observa- 

 tions regarding the structure of the distal end of the rhabdosome 

 and the growth of the common canal. Both authors assert 

 that the formation of the common canal goes hand in hand 

 with the budding of the theciB. Such a mode of growth 



* S. L. Torqunlst. Observations on the structure of some Diprionidre. Lund's Univ. Arsskrift, Bd. 

 29, Lund. 1892-93. 



