PERIGONIMUS. 89 



form, expand gradually from the base to the top, and do 

 not present the cup-shaped enlargement above, which 

 is characteristic of Perigonimus repens, a near ally. The 

 sanded surface is also a good distinctive mark. The gono- 

 phores are generally two in number, but three sometimes 

 occur ; they are constant in position, and produce an 

 immense number of ova. 



Hub. On the undersides of stones and the roots of 

 Laminariae at Tynemouth and Cullercoats, occasionally 

 (J. A.) : Largo (T. S.W.) : under stones, and on Laminaria- 

 roots, Filey Brigg, very common (T. H.). 



Genus PERIGONIMUS, Sars. 



Der. Trepl, all round, and yovifios, fruit-bearing. 

 ATRACTYLIS, T. S. Wright (in part). 



GENERIC CHARACTER. Coenosarc sheathed in a chitinous 

 polypary ; stems branching or simple, rooted by a thread-- 

 like stolon ; polypites fusiform, with a single verticil of fili- 

 form tentacles round the base of a conical proboscis ; gono- 

 phores developed from the ccenosarc. 



Gonozooids free and medusiform. Umbrella (at the 

 time of liberation) deep bell-shaped ; manubrium short ; 

 radiating canals 4 ; marginal tentacles 2 or 4 (increasing 

 in number with age], springing from non-ocellated bulbous 

 bases. 



A GROUP of minute zoophytes of simple habit, or with 

 very slight branching, which usually colonize the shells and 

 opercula of various univalve mollusks. If we except P. 

 linearis, which is doubtfully referred to this genus, all the 

 species produce gonozooids that are nearly identical in 

 external appearance. 



