Dr. 0. Herrmann — Distribution of Graptolites. 457 



to the disc,^ states that it consists of two lamin3e, which are not 

 united to one another in the central part ; and he is of opinion that 

 this space was occupied by the soft part of the animal body. 



The central disc, as implied in its name, possesses a disc-like form, 

 in the centre of which the sicula, the starting-point of the liydro- 

 soma, is situated (Fig. 1). The margin of the disc is incurved 

 between every two branches, so that the many-branched specimens 

 present the aspect of an open umbrella. The diameter is of very 

 different lengths. In specimens with long branches it may be very 

 small, while in other, short-branched examples, it may be dis- 

 proportionately large. The length of the diameter apparently stands 

 in no regular proportion to the length of the branches, i.e. therefore 

 to the age of the individuals. In several cases the formation of the 

 disc has only just commenced in nearly mature specimens. 



Nevertheless it appears distinctly from the observations that the 

 disc can acquire greater dimensions the greater the number of rami- 

 fications, and that in the many-branched forms the disc is far more 

 rarely wanting than in those with a smaller number of branches. 

 As we find individuals in various stages of development in the Grap- 

 tolite-shales, we can sketch a tolerably distinct picture of the course 

 of development of the disc. 



In individuals which have died in their earliest yonth, nothing in 

 the shape of a disc is to be recognized. In these, no secretion of the 

 disc-substance had as yet commenced, or the central expansion, if 

 present, was deficient in preservable hard structures. The first 

 traces of the formation are perceptible in somewhat more mature 

 specimens on the margin of the branches in the vicinity of the 

 sicula. This stage of development is represented by forms in which, 

 in the compressed state, we see a thin membrane stretched between 

 the forks of the branches, that is to say, around the transverse beam 

 uniting the two halves of the hydrosoma, and on the proximal parts 

 of the 8, 12, or more branches. 



The secretion advances further. The periphery of the disc 

 increased ; the disc-substance must have become thickest in the 

 neighbourhood of the centi'e, and gradually diminished in strength 

 towards the margin. We consequently meet with older individuals 

 in which the central part of the disc is quite opaque, while the 

 marginal parts are still delicate and translucent. 



As the disc attained its complete development, this difference in 

 .strength disappeared gradually, the marginal parts acquiring a 

 uniformly greater thickness by the continual advance of the 

 substance outwards from the centre ; and in fact, in its most perfect 

 development, the disc forms a plate of uniform thickness. 



But as a secretion of chitine could not proceed from the solid 

 margins of the branches, but, as zoology teaches us, only from soft 

 body-substance, we must assume that the disc, as indeed Hall 

 supposes, was filled with soft substance in its central part. But how 

 this was connected with the coeuosarc of the polypary has not yet 

 been ascertained. 



' Geological Survey of Canada : Graptolites of the Quebec Group, Montreal, 1865, 

 and Introduction to the Study of the Graptolites, Albany, 1868. 



