Dr. Gerhard Holm — On the structure of GraptoUtes. 435 



General revieio of the structure of tlie proximal part of the poly pari/ 

 (Terminology). — For the sake of conciseness and clearness in 

 description, and an easier comparison between different forms, and 

 also for greater precision in the terms employed, many of which 

 have been only lately introduced, there follows here a general 

 description of the proximal end in the genera described below. The 

 terms have been adapted as far as possible so that they can be also nsed 

 for the other groups of Graptolites which are provided with a sicula. 



The siciila, which has a cone-like form, is defined by Lapworth ^ 

 as " The chitinous covering of the free zooid ' germ ' or embryo in 

 the Graptoloidea." As Wiman has shown in Diplograptus, sp., and 

 in M. dubius, Suess, the sicula is composed of two essentially 

 different parts, the proximal and the distal, distinguished by the 

 different nature of the periderm. In the proximal or more pointed 

 end of the sicula, or, as it may perhaps be better termed, the initial 

 part, the periderm-wall is very thin and pellucid, and provided with 

 longitudinal branching and anastomosing swellings or strise, which 

 disappear near the boundary with the distal part, but which, uniting 

 together at the apex of the sicula, form the projecting virgula. 

 In the wider or distal (or perhaps better termed apertural) portion 

 of the sicula, the periderm is thicker and less pellucid, and it is 

 provided with lines of growth like those of the rest of the polypary. 

 In opposition to Wiman, I believe that the more pointed end of the 

 sicula, which has no growth-lines, exactly corresponds to the original 

 " chitinous covering of the free zooid germ or embryo," and thus is 

 the " initial part " from which the " apertural part " is developed 

 which everywhere shows normal lines of growth. 



The apertural part of the sicula, as far as can be judged, has the 

 same function as a theca, and might therefore be justly considered 

 as the first theca of the polypary (see fig. 1). The old name 

 '' sicula " is nevertheless a convenient and significant one, and is 

 here retained. Even if there be some doubt whether the whole 

 apertural part of the sicula should be regarded as the first theca, 

 it cannot at least be extended to that part of the same which lies 

 below the passage opening into the second theca. It is evident from 

 the direction of the lines of growth at this spot in Diplograptus, sp., 

 and M. dubius, Suess, that this connection with the growth of the sicula 

 first appears as a notch in the apertural margin, which afterwards by 

 further outward growth of the edge of the sicula is changed into a 

 foramen closed all round. The common canal, by which all the cells 

 of the polypary are connected with one another — whether the 

 different individuals have been developed by budding from the 

 coenosarc, or whether they are developed from and connected with 

 the one immediately preceding — (the common canal) must therefore 

 be considered to begin already in the sicula, even if it has been con- 

 venient, for the sake of description, to distinguish one part as the 

 " connecting " canal. There does not exist any important morpho- 

 logical difference whatever between the part of the sicula lying 

 below the passage opening to the second theca and the remaining 

 thecse. 



1 Quart. Journ. Geol. Soc, vol. xxxi, 1875, p. 639. 



