332 ESCHAllID^E. 



on the contrary, arc usually much more delicate, their 

 branches are comparatively slender, and the whole zoa- 

 rium has a lightness and elegance which are wanting 

 in the ordinary Cornish form. The branches in this 

 species arc, as a rule, decidedly compressed, and the 

 ramification is, for the most part, irregular and intricate. 

 In a Shetland variety, however, which I owe to Mr. 

 Norman, the zoarium is very sparingly ramified, and the 

 branches are elongate, nearly simple and roundish. In 

 southern specimens the stem is often of great thickness 

 towards the base, measuring as much as half an inch in 

 diameter. 



The cell of P. compressa seems to be liable to very little 

 variation, except such as is connected with age and the 

 several stages of growth. Within the limits of one and 

 the same colony indeed it presents very different appear- 

 ances according to its position. Towards the growing 

 extremity of the branches the zocecia are regularly dis- 

 posed, immersed, and often punctured over the surface. 

 These portions of the zoarium, in the smoothness of their 

 surface and the regularity and neatness of their appear- 

 ance, differ widely from the rest. At a short distance 

 below the top the branches are, as it were, strewn over 

 with numbers of cells, forming a superficial layer, and 

 giving them a rough and warty appearance. These cells 

 are distributed without any regularity over the surface ; 

 they are raised, prominent, ventricose, with thickened 

 walls, and a single row of marginal punctures. They 

 extend from a little below the extremities ; over the whole 

 of the lower portions of the zoarium. 



The young cells have a plain semicircular mouth ; and 

 the formation of the raised secondary orifice, which bears 

 the avicularium on its lower margin, may be traced in 

 almost any colony. A smooth space may be noticed 



