32 



JOHN M. CLARKE 



These features of ontogeny are brought out for a number of species 

 in the accompanying figures. 



Fig. 8 



Fig. 



Fig. II Fig. 12 Fig. 13 



Figs. 8-1 i. — Various expressions of the Hydnoceras tufted type in which the 

 Prismodictya and the earlier Dictyospongia stages are indicated. 



Fig. 12. — Hydnoceras Walcotti; an extreme expression of this combination, show- 

 ing notable retention of the Dictyospongia stage, union of the prismatic and tufted 

 stages, and the development of the annulated condition. 



Fig. 13. — Botryodictya, a condition in which the Dictyospongia stage is pro- 

 tracted and abruptly develops into a cup-shaped, tufted, and pouched condition. 



. In many species ontogenetic development is carried into 

 extremes of nodosity and annulation, resulting, especially in later 

 species, in great variabihty of form even within the confines of the 

 Devonian. 



