38 G. F. MATTHEW ON THE 



G-KOWTH AND DEVELOPMENT. 



The growth and development of Dictyonema is full of interest to the biologist. 

 When it budded from the sicula it developed two upright-growing branches, which in 

 their turn branched again without showing the characteristic cross-bars which united 

 the later branches of the hydrosome. It is on these tertiary branches that we first see the 

 connecting threads that held together the circle of branches ; hence Dictyonema began 

 its growth as a Dichograptid, with the hydrothecse turned inward toward each other on 

 the branches. But the branching was not always so regular as the above remarks might 

 imply, for sometimes while about half the branches branched simultaneously, the other 

 half would continue to grow without branching for some time longer ; these colonies 

 then would take the form of Bryograptus, before assuming the generic characters of 

 Dictyonema. 



From the tertiary branches upwardlthe branches of the hydrosome are connected by 

 transverse filaments, which at first are far apart, but soon become numerous enough to 

 form the typical net of a Dictyonema. At first the branches are far from parallel, and, 

 being few, have sometimes been flattened on their sides in the shale, thus exposing the 

 hydrothecae to view. In the upper part of the rods the serratures are rarely seen, owing 

 to the fact that they are there turned inward from the outer face of the hydrosome. Near 

 the sides of the hydrosome, where the branches are more liable to be pressed on their 

 sides, the hydrothecœ sometimes show themselves on the outer, sometimes on the inner 

 side of the branch. The inward orientation of the cells is very manifest in the primary 

 and secondary branches, showing the close relation of this genus with Dichograptus and 

 Bryograptus. 



Bryograptus is known to occur just above the horizon which carries Dictyonema 

 flabelliforme. Two species have been described from the Swedish beds and others from 

 Great Britain. A more slender form of Dichograptid, which by If. A. Nicholson has been 

 referred to Trichograptus, and by O. Hermann to Clonograptus, was described by J. G-. 0. 

 Linnarsson, some years ago, from beds containing Sphcerophihalmus alalus. This therefore 

 should have been somewhat older than the true Dictyonema beds. Of one of the species 

 which is found just above the Dictyonema beds, B. KJerulfi, Dr Brôgger says that it 

 appeared to be branched like a bush, and so would resemble the initial branches of 

 Dictyonema. 



One writer suggests that the lower irregular part of the hydrosome in Dictyonema 

 was a sort of root or support in the form of a cage buried in the mud by which the colony 

 was anchored, but such a view does not seem tenable, as the whole of these lower branches 

 from the sicula upward, are edged with cells or hydrothecae, and the sicula itself is too 

 small to have given any support. From the way in which the hydrosomes are scattered 

 over the layers of the shale, it would rather seem that the colonies were free to float 

 through the water and sink to the bottom when their term of existence was over.' While 

 some layers are covered with entire hydrosomes, others are besprinkled with the fragments 

 of broken colonies, mingled with minute budding siculse or young polyparies in the first 

 stages of growth. In these young examples the branches are often obscured or concealed 

 by a (chitenous ?) growth, perhaps analogous to the disc in some of the Dichograptidse. 



' Salter's figure ('Mem. Geol. Surv. G. B.,' vol. iii, pi. iv, fig. 1) is not a natural representation of the way in 

 which this species is scattered over the layers of shale. 



