HYDROZOA. GRAPTOLITOIDEA. 43 



the genus Dicranograptus, however, there is a transition 

 between these two types, its proximal end being diprioni- 

 dian, whilst its distal end is monoprionidian. The hydro- 

 thecse vary considerably in different genera, and even in 

 different species of the same genus; but in the same 

 species they are similar, except that they diminish in size 

 towards the proximal end of the polypary ; they may be 

 rectangular (e.g. Phyllograptus), tubular (e.g. Rastrites), 

 conical (e.g. Monograptus spiralis), or recurved (e.g. Mono- 

 graptus lobiferus). They may be in contact throughout 

 their entire length (e.g. Phyllograptus), at their bases only 

 (e.g. Ccenograptus), or entirely separate (e.g. Rastrites). 

 Frequently they are provided with one or more spines 

 near the mouth. By most authors the hydrothecse are 

 said to communicate freely with the common canal, in 

 this respect differing from the allied living forms, in which 

 there is a constriction, or a diaphragm, at the base of each 

 hydrotheca separating it from the common canal. But 

 McCoy and Hopkinson state that in some specimens, 

 as for instance Didymograptus extensus, Didymograptus 

 patulus, and Tetragraptus serra, from the Skiddaw Slates, 

 there is a distinct septum between the hydrothecse and 

 the common canal, and moreover in some cases the com- 

 mon canal itself is divided up by transverse septa. 



The periderm of the graptolite is said by some authors 

 to be formed of three layers. In the genus Retiolites the 

 two inner layers are modified so as to form a fibrous net- 

 work, upon which the outer layer is stretched as a kind of 

 epiderm. 



We saw that in Monograptus, the virgula sometimes 

 extends beyond the distal end of the polypary ; in some 

 genera (e.g. Diplograptus) it sometimes also extends beyond 

 the proximal extremity. In the majority of cases the vir- 



