530 Prof. Nicholson 8f J. E. Marr — Phytogeny of the GraptoUtes. 



in the fact that the proximal aperture gives connection with the 

 coenosarc.^ Among these forms there are three more important 

 types of hydrotheca3. In one gronp, the hj^drothecse are narrow 

 and straight, the neighbouring cells barely overlapping, or not 

 overlapping at all, while the aperture is straight, and is usually 

 at right angles to the back of the stipe. In a second group, the 

 hydrothecse are also straight, but overlap to a marked extent, while 

 the aperture is straight, and is at right angles to the back of the 



Group 1. 



Bryograptus 



Callavei 



Group 3. 



B. ramosus 



Tetragraptus 



Hicksii 



T. f ruticosus 



Dibymograptus 



aff inis 



D. Murchisoni 



stipe or at right angles to the hydrothecsB. In a third group, the 

 bydrothecEe are curved, narrow at their bases and widening towards 

 their mouths, while the aperture is curved, and its upper lip is 

 usually prolonged into a pointed process or spine. In contrast 

 with the comparatively simple hydrothecse of the older types of 

 Graptolites, we find later types, such as Dicellograptus, Dicrano^ 

 graptus, and more especially Monograptus, generally characterized by 

 ' See Holm, Geol. Mag., Dec. IV, Vol. II, p. 435. 



