166 PBOF. W. N. BENSON AND DK. S. SMITH ON [vol. lxxix, 



third of the diameter of the corallite. In its microscopical structure 

 the columella is very similar to that of Amygdalopliyllwm (p. 162). 

 Within this solid axis there is buried a short medial plate, and 

 the inner ends of the major septa which extend to the plate. 

 Very occasionally, additional lamellae are inserted between the 

 embedded ends of the major septa ; these lamellae correspond in 

 position to the minor septa, although the two are not connected. 



The general structure of the columella is exhibited fairly well 

 by the corallite lettered d in PL VIII, fig. 5 ; but in most corallites 

 recrystallization has obscured the structure to some extent. 



Tabulae. — The tabulae, for the greater part, extend from the 

 theca to the columella, but are bent irregularly and at high angles 

 both towards the theca and towards the columella (PL IX, fig. 7). 

 There is no great development of tabellae like that seen in the 

 Litliostrotion from the same region. 



Ontogeny. 



In the figure of Gionodendron (PL VIII, fig. 5) several stages 

 in the development of the corallite are illustrated : these are 

 lettered a, b, c, and d. 



a (1 mm. in diameter) represents a very early neanic stage. 

 There are only about twelve major septa present, and one 

 of these — the cardinal or the counter-septum — is dilated to 

 form a conspicuous columella ; the other septa join the 

 columella, as in later stages. The minor septa are very 

 short, and the dissepimental tissue is undeveloped. The 

 pinnate symmetry is distinct. 



b (2 mm. in diameter) shows the corallite at a later neanic 

 stage. The septa have increased in number, the minor 

 septa are well developed, and an extrathecal area is added. 

 The columella is not conspicuously large or well formed, 

 and at this stage Gionodendron is not markedly differ- 

 entiated from Litliostrotion. A pinnate symmetry is still 

 discernible. 



c (3 mm. in diameter). The corallite has practically attained 

 its ephebic characters, although it has only reached half 

 (or little more than half) of its mature dimensions. The 

 inner ends of the major septa have gradually been enclosed 

 by an expanding columella. 



d. Ephebic stage. 



Affinity and Comparison. 



Gionodendron columen is a species of the Lithostrotiontidae ; 

 but, in view of the character of the columella, we may fairly assign 

 to it generic rank, and so mark it off: from the more usual forms 

 of Litliostrotion. Apart from the columella, and in a less degree 

 in the character of the tabulae, its form and structures are essenti- 

 ally those of Litliostrotion. As may be expected, it resembles more 



