SCHIZOPORELLA VENUSTA. 277 



The zooecia are comparatively short in the older portions 

 of the colony, and more elongated towards the margin. 

 The shape of the orifice is very peculiar and distinctive : 

 the portion above the hinge of the oral valve is round ; but 

 at this point there is a slight constriction, caused by two 

 small lateral projections; immediately below them the 

 margin recedes, and two minute indentations are formed, 

 and the aperture then narrows away into a shallow and 

 somewhat pointed sinus. The whole orifice has an elon- 

 gated appearance, and tapers off downwards, the regularity 

 of its outline being broken by the lateral sinuses. 



The central umbo is of large size, and seems to be 

 generally present. Above the cells there is a small rect- 

 angular space, marked off like the ordinary zooecia, and, 

 like them, with a punctured surface, but having a minute 

 avicularium near the top of it in the place that would 

 be occupied in a normal cell by the oral aperture. When 

 the ovicell is present, it covers a large portion of this 

 avicularian cell, but the avicularium itself projects beyond 

 it and is visible on the summit. 



I have met with a considerable group of these modified 

 cells, of various sizes and shapes, in the midst of a colony ^ 

 and on one or two of them the umbo was present in the 

 usual position (Plate XXX. fig. 7). They afford one more 

 striking illustration of the morphological relation between 

 the avicularium and the zooscium. 



The remarkable structure of the orifice of the cell in 

 the present form seems at first sight to separate it in a 

 very decisive way from the allied species. But it must be 

 remembered that we have the same peculiarity, though in 

 a much less marked degree, in S. sanguined, a species 

 which in general character agrees very closely with 5. 

 linearis. In what seems to be a variety of the latter, I 

 have noticed an approach to the samo condition ; whilst in 



