56 HYDROZOA. GRAPTOLITOIDEA 



ORDER III. GRAPTOLITOIDEA 



The graptolites are found only in the Lower Palaeozoic 

 rocks, where, owing to their abundance and to the limited 

 range in time of both genera and species, combined with 

 their wide geographical distribution, they are of great 

 importance to the stratigraphical geologist. They occur 

 most commonly in argillaceous rocks, especially in black 

 shales, whilst they are rare in sandstones and limestones. 

 The graptolites resemble the Calyptoblastea, e.g. Sertu- 

 laria and Plumidaria; they were compound animals, 

 and the soft parts were protected by a skeleton of chitin. 

 But the original material of the skeleton is seldom preserved 

 unaltered ; in some cases it has been replaced by iron 

 pyrites, but usually it has become carbonised. 



The entire skeleton (exclusive of the soft parts) is 

 termed the polypary; this in an unbranched form like 

 Monograptus consists of a tubular part known as the 

 common canal (fig. 13 b, c), which extends nearly the 

 whole length of the animal, the wall being termed the 

 periderm or perisarc. From one side of the common canal 

 small tooth-like projections are given off; these are the 

 hydrothecod (fig. 13 b, h), each of which is hollow and 

 opens on the one hand into the common canal and on the 

 other to the exterior ; the latter aperture, known as the 

 mouth (m) of the hydrotheca, is frequently circular, but 

 sometimes quadrangular or slit-like. Embedded in the 

 periderm on the side opposite to the row of hydrothecaB 

 is a chitinous thread or rod, termed the virgida (fig. 13 b, a). 

 In some species of Monograptus the virgula projects 



