THE LOWER SILURIAN PERIOD. 101 



Passing next to the sub-kingdom of Ccclenterate animals 

 (Zoophytes, Corals, &c.), we find that this great group, almost 

 or wholly absent in the Cambrian, is represented in Lower 

 Silurian deposits by a great number of forms belonging on the 

 one hand to the true Corals, and on the other hand to the 

 singular family of the Graptolites. If we except certain plant- 

 like fossils which probably belong rather to the Sertularians 

 or the Polyzoans {e.g., Dictyonema, Dendrograptus, &c.)> the 

 family of the Graptolites may be regarded as exclusively 

 Silurian in its distribution. Not only is this the case, but it 

 attained its maximum development almost upon its first ap- 

 pearance, in the Arenig Rocks ; and whilst represented by a 

 great variety of types in the Lower Silurian, it only exists in 

 the Upper Silurian in a much diminished form. The Grap- 

 tolites (Gr. grapho, I write; lithos, stone) were so named by 

 Linnaeus, from the resemblance of some of them to written or 

 pencilled marks upon the stone, though the great naturalist him- 

 self did not believe them to be true fossils at all. They occur 

 as linear or leaf-like bodies, sometimes simple, sometimes com- 

 pound and branched; and no doubt whatever can be enter- 

 tained as to their being the skeletons of composite organisms, 

 or colonies of semi-independent animals united together by 

 a common fleshy trunk, similar to what is observed in the 

 colonies of the existing Sea-firs (Sertularians). This fleshy 

 trunk or common stem of the colony was protected by a deli- 

 cate horny sheath, and it gave origin to the little flower-like 

 " polypites, " which constituted the active element of the whole 

 assemblage. These semi-independent beings were, in turn, 

 protected each by a little horny cup or cell, directly connected 

 with the common sheath below, and terminating above in an 

 opening through which the polypite could protrude its tentacled 

 head or could again withdraw itself for safety. The entire 

 skeleton, again, was usually, if not universally, supported by 

 a delicate horny rod or " axis, " which appears to have been 

 hollow, and which often protrudes to a greater or less extent 

 beyond one or both of the extremities of the actual colony. 



The above gives the elementary constitution of any Grapto- 

 lite, but there are considerable differences as to the manner in 

 which these elements are arranged and combined. In some 

 forms the common stem of the colony gives origin to but a 

 single row of cells on one side. If the common stem is a 

 simple, straight, or slightly-curved linear body, then we have 

 the simplest form of Graptolite known (the genus Monograptus) ; 



