40 



Mr. Hopkinson prepared a paper specially on this subject, and 

 read it before the British Association at York on the 7th of 

 of September, 1881. This paper appeared, in an abridged form, 

 in the Geological Magazine for October, 1881, and in the Annals 

 and Magazine of Natural History for January, 1882. Mr. Hop- 

 kinson does not suppose that the cellules were entirely cut off 

 from the common canal, or that the division of the coenosarc into 

 sections was complete, he speaks of the septa as strictures. Professor 

 Lapworth also speaks of them in the same way ; he compares the 

 septa to the hard joints in reeds or tall stems of grass, by which 

 they are divided into sections, a minute aperture only being left for 

 the passage of nourishment. 



When Graptolites were living organisms, there can be no doubt 

 that each cellule contained a zooid, which was connected, however 

 slightly, with the general body; and that this zooid, like the 

 Sertularian Zoophytes of the present time, would be furnished with 

 tentacles or arms, for the purpose of securing its food. Also 

 that it would probably possess the power to protrude its head 

 beyond the mouth of the cellule, when necessary, and retire 

 altogether within it when an enemy approached. 



The form of the cell-mouth, so far as it can be ascertained, was 

 usually round, and it opened generally in the direction of the 

 cellule, but in some cases obliquely, and in others it was so bent 

 as to open directly towards tjie base or proximal end of the 

 polypary (Fig. 7). 



The most simple classification of the Graptolite family is that 

 adopted by Professor Nicholson, according to whom the true 

 Graptolites are divided into three groups, namely, the Monoprio- 

 nidian group, or forms with a single row of cellules; the 

 Diprionidian group, with two rows of cellules ; and the Tetraprio- 

 nidian group, with four rows of cellules. There is also another 

 group, the Dendroidea, which consists of much-branched and 

 plant-like forms. Amongst the forms of which the true nature js 

 doubtful, but which occur in the same beds, and bear more or less 

 likeness to the Graptolite family are here placed under the heading 



